Understanding the Formation and Evolution of Disc Break Features in Galaxies

UNDERSTANDING THE FORMATION AND EVOLUTION OF DISC BREAK FEATURES IN GALAXIES

JARKKO LAINE

Astronomy Research Unit University of Oulu Finland

Academic Dissertation to be presented with the assent of the Faculty of Sci- ence, University of Oulu, for public discussion in the Auditorium GO101, on September 16th, 2016, at 12 o’clock.

REPORT SERIES IN PHYSICAL SCIENCES Report No. 101 OULU 2016 • UNIVERSITY OF OULU Opponent Prof. John Beckman, Instituto de Astrof´ısicade Canarias, La Laguna, Spain

Reviewers Prof. Jari Kotilainen, Finnish Centre for Astronomy with ESO, University of Turku, Finland Dr. Ruym´anAzzollini, Mullard Space Science Laboratory, United Kingdom

Custos Docent Eija Laurikainen, Astronomy Research Unit, University of Oulu, Finland

ISBN 978-952-62-1303-3 ISBN 978-952-62-1304-0 (PDF) ISSN 1239-4327 Juvenes Print Oulu 2016 i

Laine, Jarkko Johannes: Understanding the formation and evolution of disc break features in galaxies Astronomy Research Unit P.O. Box 3000 FIN-90014 University of Oulu FINLAND

Abstract

The discs in galaxies are radially extended, rotationally supported, flattened systems. In the cosmological Lambda Cold Dark Matter model the formation of the discs is intimately connected with galaxy formation. Generally it is assumed that the discs have exponentially decreasing stellar surface brightness profiles, but completely satisfactory theoretical explanation for this has not been presented. Large number of studies in the past decade have challenged this view, and have found a change in the slope of the surface brightness profile in the outer regions of many galaxies discs: the surface brightness can decrease more, or less, steeply than in the inner regions. The transition between the two slopes is often called a disc break. Consequently, the discs are divided in three major categories: single exponential Type I, down-bending break Type II, and up-bending break Type III. Formation of these break features has been linked to the initial formation of the discs, internal evolution, and also with the interactions between galaxies. By studying the detailed properties of the disc break features, the evolutionary history of discs, and galaxies in general, can be better understood. The thesis work focuses on the structural analysis of the galaxies in the Spitzer Survey of Stellar Structure in Galaxies (S4G), which consists of 2352 galaxies observed in the 3.6 and 4.5 µm mid-infrared wavelengths with the Spitzer space telescope. Work has been carried out as a part of the data- analysis pipelines of the S4G survey, utilizing surface photometry. In addition, special emphasis has been put on the study of the disc and disc break properties in a wide range of galaxy morphological types and stellar masses. The thesis work attempts to at least partially understand how galaxy stellar mass and observed wavelength affect the properties of the discs and breaks, and how galaxy structural components are connected with the breaks. The data comprises mainly of the 3.6 µm infrared data, providing a view to the stellar mass distribution of galaxies. We find that the Type II breaks are the most common disc profile type, found in 45 ± 2% of the sample galaxies, consisting of 759 galaxies in the stellar mass range 8.5 . log10(M∗/M ) . 11. Type I discs are found in 31 ± 2%, and the Type III breaks in 23 ± 2% of the sample. The fraction of the profile types also depends of the galaxy stellar mass: fractions of the Types II and III ii increase, while Type I fraction decreases, with increasing stellar mass. We attribute these changes with stellar mass to the increased frequency of bar resonance structures in higher mass galaxies, which are commonly associated with a Type II break, and to the increased fraction of Type III profiles in generally more massive early-type disc galaxies. In addition to the Type II breaks associated with bar resonance structures, we find that nearly half of these breaks relate to the visual spiral outer edge, confirming previous results of the Type II break connection with galaxy structure, and thus the internal evolution rather than initial formation of discs. Complementary data in optical wavelengths from the Sloan Digital Sky Survey shows a strong change in the properties of the discs inside the Type II breaks, indicating that the inner discs are evolving via star formation. In late- type spiral galaxies (T & 4) with a Type II break, possible evidence of radial stellar migration is found in the outer disc: the slope of the surface brightness profile is shallower in the infrared compared to optical wavelengths, indicating that older stellar populations are more evenly spread throughout the disc. Formation of the Type I and III profiles remain poorly understood. However, indication that some of the Type III profiles are formed by environmentally driven processes is found, with a correlation between the properties of the local environment and the disc profile parameters. Furthermore, indication of star formation possibly causing the up-bends in spiral galaxies is found through a presence of young stellar population in the outer disc section.

Key words: Galaxies, galaxy formation, galaxy evolution, galaxy environment, photometry, infrared, optical Acknowledgments

The work presented in this PhD thesis has been performed at the Astronomy Research Unit in the University of Oulu. First of all I would like to thank the supervisors of the thesis work, Docent Eija Laurikainen and Professor Heikki Salo, for your support and guidance throughout these past years. Without your tremendous help this work would not have been possible. I thank you also for introducing me to the S4G and other collaborations, which have provided unique opportunities. Special thanks goes to the past and present fellow PhD students of the galaxy group: SimónD´ıaz-Garc´ıa,Mart´ınHerrera-Endoqui, Marius Lehmann, Aku Venhola, and Joachim Janz, for your company, support, and help during these years. I am also grateful for all the other members of the astronomy unit: Pertti Rautiainen, SébastienComerón,JürgenSchmidt, Xiaodong Liu, Vitaly Neustroev, Soile Kukkonen, and Tuulia Pennanen, for creating a pleasant and supportive atmosphere to work in. I would also like to thank Prof. Jari Kotilainen and Dr. RuymánAzzollini for the pre-examination of this thesis. Many others are also deserving of my gratitude, however they are too numerous to be listed here. Financial support from the Vilho, Yrjö,and Kalle Väisäläfoundation of the Finnish Academy of Science and Letters, Finnish Academy, Physics Department of University of Oulu, and University of Oulu Graduate School is also acknowledged. Finally, I thank my friends near and far for all your time and company, which has always reminded me of what is truly important in life. I am deeply grateful for my parents and family. You have always allowed me to find my own way, and have supported me in whichever path I choose.

Oulu, August 2016 Jarkko Laine

iii iv LIST OF ORIGINAL PAPERS

The present thesis contains an introductory part and the following papers, referred in the text by their Roman numerals:

I. Laine J., Laurikainen E., Salo H., ComerónS., Buta R. J., Zaritsky D., Athanassoula E., Bosma A., Muñoz-Mateos J.-C., Gadotti D. A., Hinz J. L., Erroz-Ferrer S., Gil de Paz A., Kim T., Menéndez-Delmestre K., Mizusawa T., Regan M. W., Seibert M., Sheth K. Monthly Notices of the Royal Astronomical Society, 2014, 441, 3: Morphology and environment of galaxies with disc breaks in the S4G and NIRS0S

II. Muñoz-MateosJ.-C., Sheth K., Regan M., Kim T., Laine J., Erroz- Ferrer S., Gil de Paz A., Comeron S., Hinz J. L., Laurikainen E., Salo H., Athanassoula E., Bosma A., Bouquin A. Y. K., Schinnerer E., Ho L., Zaritsky D., Gadotti D. A., Madore B., Holwerda B., Menéndez- Delmestre K., Knapen J. H., Meidt S., Querejeta M., Mizusawa T., Seibert M., Laine S., Courtois H. The Astrophysical Journal Supplement Series, 2015, 219, 1: The Spitzer Survey of Stellar Structure in Galaxies (S4G): Stellar Masses, Sizes, and Radial Profiles for 2352 Nearby Galaxies

III. Salo H., Laurikainen E., Laine J., ComerónS., Gadotti D. A., Buta R. J., Sheth K., Zaritsky D., Ho L., Knapen J., Athanassoula E.; Bosma A., Laine S., Cisternas M., Kim T., Muñoz-MateosJ.-C., Regan M. W., Hinz J. L., Gil de Paz A., Menéndez-Delmestre K., Mizusawa T., Erroz-Ferrer S., Meidt S. E.. Querejeta M. The Astrophysical Journal Supplement Series, 2015, 219, 1: The Spitzer Survey of Stellar Structure in Galaxies (S4G): Multi-component Decomposition Strategies and Data Release

IV. Laine J., Laurikainen E., Salo H. Astronomy & Astrophysics, submitted: Inﬂuence of galaxy stellar mass and observation wavelength on disc breaks v

Author contributions All the papers included in this thesis are results of teamwork. In papers I and IV the author performed the data-analysis and was the primary writer. In paper II the author has participated in large part of the data-analysis and writing of chapter 4, and has contributed to the data-analysis described in the other chapters. In paper III the author has a strong contribution to the data-analysis (> 20%) and has participated in the planning of the methods used in the study. In addition, the author has participated in the work of following papers which are not included in this thesis:

1. Zaritsky, D., McCabe, K., Aravena, M., Athanassoula, E., Bosma, A. et al. The Astrophysical Journal, 2016, 818, 1: Globular Cluster Populations: Results Including S4G Late-type Galaxies

2. Querejeta, M., Meidt, S. E., Schinnerer, E., Cisternas, M., Mu˜noz-Mateos, J. C. et al. The Astrophysical Journal Supplement Series, 2015, 219, 1: The Spitzer Survey of Stellar Structure in Galaxies (S4G): Precise Stellar Mass Distributions from Automated Dust Correction at 3.6 µm

3. de Swardt, B., Sheth, K., Kim, T., Pardy, S., D’ Onghia, E. et al. The Astrophysical Journal, 2015, 808, 1: The Odd Oﬀset between the Galactic Disk and Its Bar in NGC 3906

4. Buta, R. J., Sheth, K., Athanassoula, E., Bosma, A., Knapen, J. H. et al. The Astrophysical Journal Supplement Series, 2015, 217, 2: A Classical Morphological Analysis of Galaxies in the Spitzer Survey of Stellar Structure in Galaxies (S4G)

5. Bouquin A. Y. K., Gil de Paz A., Boissier S., Mu˜noz-MateosJ.-C., Sheth K. et al. The GALEX/S4G UV-IR Color-Color Diagram: Catching Spiral Galaxies Away from the Blue Sequence

6. Zaritsky, D., Aravena, M., Athanassoula, E., Bosma, A., Comer´on,S. et al. The Astrophysical Journal, 2015, 799, 2: Globular Cluster Populations: First Results from S4G Early-type Galaxies

7. Kim, T., Sheth, K., Gadotti, D. A., Lee, M. G., Zaritsky, D. et al. The Astrophysical Journal, 2015, 799, 1: The Mass Proﬁle and Shape of Bars in the Spitzer Survey of Stellar Struc- ture in Galaxies (S4G): Search for an Age Indicator for Bars vi

8. Laine, S., Knapen, J. H., Mu˜noz-Mateos, J.-C., Kim, T., Comer´on,S. et al. Monthly Notices of the Royal Astronomical Society, 2014, 444, 4: Spitzer/Infrared Array Camera near-infrared features in the outer parts of S4G galaxies

9. Meidt, S. E., Schinnerer, E., van de Ven, G., Zaritsky, D., Peletier, R. et al. The Astrophysical Journal, 2014, 788, 2: Reconstructing the Stellar Mass Distributions of Galaxies Using S4G IRAC 3.6 and 4.5 µm Images. II. The Conversion from Light to Mass

10. Zaritsky, D., Courtois, H., Mu˜noz-Mateos,J.-C., Sorce, J., Erroz-Ferrer, S. et al. The Astronomical Journal, 2014, 147, 6: The Baryonic Tully-Fisher Relationship for S4G Galaxies and the ”Con- densed” Baryon Fraction of Galaxies

11. Kim, T., Gadotti, D. A., Sheth, K., Athanassoula, E., Bosma, A. et al. The Astrophysical Journal, 2014, 782, 2: Unveiling the Structure of Barred Galaxies at 3.6 µm with the Spitzer Survey of Stellar Structure in Galaxies (S4G). I. Disk Breaks

12. Comer´on,S., Salo, H., Laurikainen, E., Knapen, J. H., Buta, R. J. et al Astronomy & Astrophysics, 2014, 562: ARRAKIS: atlas of resonance rings as known in the S4G

13. Holwerda, B. W., Mu˜noz-Mateos,J.-C., Comer´on,S., Meidt, S., Sheth, K. et al. The Astrophysical Journal, 2014, 781, 1: Morphological Parameters of a Spitzer Survey of Stellar Structure in Galax- ies

14. Elmegreen, D. M., Elmegreen, B. G., Erroz-Ferrer, S., Knapen, J. H., Teich, Y. et al. The Astrophysical Journal, 2014, 780, 1: Embedded Star Formation in S4G Galaxy Dust Lanes

15. Cisternas, M., Gadotti, D. A., Knapen, J. H., Kim, T., D´ıaz-Garc´ıa,S. et al. The Astrophysical Journal, 2013, 776, 1: X-Ray Nuclear Activity in S4G Barred Galaxies: No Link between Bar Strength and Co-occurrent Supermassive Black Hole Fueling

16. Zaritsky, D., Salo, H., Laurikainen, E., Elmegreen, D., Athanassoula, E. et al. vii

The Astrophysical Journal, 2013, 772, 2: On the Origin of Lopsidedness in Galaxies as Determined from the Spitzer Survey of Stellar Structure in Galaxies (S4G)

17. Mu˜noz-Mateos,J. C., Sheth, K., Gil de Paz, A., Meidt, S., Athanassoula, E. et al. The Astrophysical Journal, 2013, 771, 1: The Impact of Bars on Disk Breaks as Probed by S4G Imaging

18. Erroz-Ferrer, S., Knapen, J. H., Font, J., Beckman, J. E., Falc´on-Barroso, J. et al. Monthly Notices of the Royal Astronomical Society, 2012, 427, 4: Hα kinematics of S4G spiral galaxies - I. NGC 864

19. Mart´ın-Navarro, I., Bakos, J., Trujillo, I., Knapen, J. H., Athanassoula, E. et al. Monthly Notices of the Royal Astronomical Society, 2012, 427, 2: A uniﬁed picture of breaks and truncations in spiral galaxies from SDSS and S4G imaging

20. Comer´on,S., Elmegreen, B. G., Salo, H., Laurikainen, E., Athanassoula, E. et al. The Astrophysical Journal, 2012, 759, 2: Breaks in Thin and Thick Disks of Edge-on Galaxies Imaged in the Spitzer Survey Stellar Structure in Galaxies (S4G)

21. Kim, T., Sheth, K., Hinz, J. L., Lee, M. G., Zaritsky, D. et al. The Astrophysical Journal, 2012, 753, 1: Early-type Galaxies with Tidal Debris and Their Scaling Relations in the Spitzer Survey of Stellar Structure in Galaxies (S4G)

22. Meidt, S. E., Schinnerer, E., Mu˜noz-Mateos,J.-C., Holwerda, B., Ho, L. C. et al. The Astrophysical Journal Letters, 2012, 748, 2: The S4G Perspective on Circumstellar Dust Extinction of Asymptotic Gi- ant Branch Stars in M100

23. Meidt, S. E., Schinnerer, E., Knapen, J. H., Bosma, A., Athanassoula, E. et al. The Astrophysical Journal, 2012, 744, 1: Reconstructing the Stellar Mass Distributions of Galaxies Using S4G IRAC 3.6 and 4.5 µm Images. I. Correcting for Contamination by Polycyclic Aromatic Hydrocarbons, Hot Dust, and Intermediate-age Stars

24. Comer´on,S., Elmegreen, B. G., Knapen, J. H., Salo, H., Laurikainen, E. et al. viii

The Astrophysical Journal, 2011, 741, 1: Thick Disks of Edge-on Galaxies Seen through the Spitzer Survey of Stellar Structure in Galaxies (S4G): Lair of Missing Baryons?

25. Comer´on,S., Elmegreen, B. G., Knapen, J. H., Sheth, K., Hinz, J. L. et al. Astrophysical Journal Letters, 2011, 738, 2: The Unusual Vertical Mass Distribution of NGC 4013 Seen through the Spitzer Survey of Stellar Structure in Galaxies (S4G)

26. Elmegreen, D. M., Elmegreen, B. G., Yau, A., Athanassoula, E., Bosma, A. et al. The Astrophysical Journal, 2011, 737, 1: Grand Design and Flocculent Spirals in the Spitzer Survey of Stellar Struc- ture in Galaxies (S4G)

27. Sheth, K., Regan, M., Hinz, J. L., Gil de Paz, A., Men´endez-Delmestre, K. et al. Publications of the Astronomical Society of Paciﬁc, 2010, 122, 898: The Spitzer Survey of Stellar Structure in Galaxies (S4G) Contents

Abstract i

Acknowledgments iii

List of original papers iv

Contents ix

1 Introduction 1 1.1 Galaxy morphology ...... 1 1.2 Disc breaks as probes of galaxy evolution ...... 3

2 Galaxy formation and evolution 5 2.1 Structure formation in the Universe ...... 5 2.2 Galaxy formation ...... 7 2.3 Issues of galaxy formation models ...... 9 2.4 Galaxy evolution ...... 12 2.4.1 External evolution ...... 12 2.4.2 Internal evolution ...... 17

3 Diﬀerent types of galaxy discs 25 3.1 Surface brightness proﬁles ...... 25 3.2 Exponential discs ...... 27 3.3 Down-bending disc breaks ...... 29 3.3.1 Edge-on versus face-on ...... 29 3.3.2 Down-bending breaks in face-on galaxies ...... 32 3.4 Up-bending disc breaks ...... 39 3.5 Open questions addressed in this thesis ...... 43

4 Data and sample selection 45 4.1 Data ...... 45 4.2 Sample selection ...... 48

ix x CONTENTS

5 Photometric decomposition methods 53 5.1 General considerations ...... 53 5.1.1 Radial proﬁle functions ...... 55 5.2 One dimensional methods ...... 57 5.2.1 Marking the disc ...... 58 5.3 Two dimensional methods ...... 63 5.3.1 Disc breaks ...... 65

6 Results 67 6.1 Disc proﬁle type fractions ...... 67 6.1.1 Comparison with other studies ...... 68 6.2 Connections with galaxy structure components ...... 72 6.3 Break properties ...... 75 6.4 Changing properties with wavelength of observations . . . . . 76

7 Conclusions and future work 81

Bibliography 84 Chapter 1

Introduction

Our knowledge of galaxies has increased by vast amounts in the past century. Just in 1920s the nature of galaxies as structures outside the Milky Way, the galaxy we reside in, was confirmed by the distance measurement of the Andromeda galaxy by Edwin Hubble. Today we know that galaxies are the basic building blocks of structure in the Universe, that most galaxies are nearly as old as the Universe, and that galaxies are complex gravitationally bound systems made of varying ratios of stars, gas, dust, and dark matter. Through the different properties of galaxies and their structure, we try to better understand the past and present processes that are responsible for their observed state. This thesis focuses on a specific structure of galaxies: the discs. These are radially extended, rotationally supported, thin, and fragile structures which show imprints of multitude of evolutionary processes that have modified them. Currently even the precise formation mechanism of the discs is not known. Nevertheless, by examining the different details of the discs, their formation and evolution can be better constrained.

1.1 Galaxy morphology

Galaxies have a large range of masses, and they appear in wide variety of different shapes. Despite the complexity of their appearance, galaxies are usually given a morphological type. The study of galaxy morphology dates back to Hubble (1936), in which study the first comprehensive galaxy classification scheme was given. This classification scheme is shown in Fig. 1.1, which is more commonly known as the Hubble tuning fork diagram. The galaxies can be divided into three classes in this scheme: elliptical galaxies, lenticular galaxies, and spiral galaxies. The disc galaxies comprise of lenticular and spiral galaxies, which can both be further divided into barred and unbarred, forming the two forks of the diagram. Through tradition, the galaxies on left of the Hubble tuning fork diagram are called “early-type”, while those in right are “late-type”. This naming implies no evolutionary

1 Figure 1.1: The classical Hubble tuning fork diagram of galaxy classification. Image credit: NASA & ESA. trend, but simplifies referring to the different types of galaxies. The very early-type galaxies, the elliptical galaxies, have generally large masses. They are further divided into E0-E7 subclasses by their apparent flatness. Elliptical galaxies have little cold gas and dust, and have red colours, indicating that they have little to no star formation. In the classical picture, these galaxies are considered to have relatively little structure. However, deep observations often reveal complex outer structures, exposing the signs of environmental processes modifying the structure of these galaxies (e.g. Duc et al. 2015; section 2.4.1). Elliptical galaxies have generally little rotation, and are instead supported by the velocity dispersion of stars in essentially random orbits. Contrary to the elliptical galaxies, the discs are rotationally supported structures, made of stars orbiting in nearly circular orbits. However, many disc galaxies also have a central concentration, a bulge, which can have a variety of different structures. The so called classical bulges physically re- semble small elliptical galaxies, being supported by velocity dispersion and

2 comprising of old stars. On the other hand, discy pseudo-bulges, for example, can possess star formation and are rotation dominated. The different types of bulges can also coexist in a galaxy. In Hubble (1936) classification scheme the disc galaxies are divided into early- and late-types by two factors: the prominence of the central concentration, and the appearance of the spiral structure. In the lenticular S0 galaxies no spiral structure is seen, while the early-type spiral Sa galaxies have tightly wound spiral arms and relatively large central concentrations. The late-type Sc spiral galaxies in the other hand have very open spiral arms and small central concentrations. A small fraction of spiral galaxies (i.e. ∼ 10%, Elmegreen and Elmegreen 1987) are dominated by two major spiral arms, known as grand-design spirals. Most spiral galaxies have multiple spiral arms, or have less well defined and patchy flocculent spiral arms. The disc galaxies also possess a large variety of masses and colours. Generally, the S0 galaxies have red colours and large masses, while the masses of spiral galaxies decrease towards the latest types. In addition, the spirals commonly have bluer colours, manifested by the recent star formation taking place in the spiral arms. Observations have shown that about two thirds of disc galaxies have a bar component in their centre. These are rotating structures in the discs, made from the disc stars which have been moved to elongated orbits by instabilities of the disc. This basic classification has been further expanded in several directions. For example, de Vaucouleurs (1959a) introduced classes for irregular and very-late type Sd spiral galaxies, in which no continuous arms are seen, as well as a sub-classification of ring structures. Several different types of bulges, and many other details have also been added to the galaxy classification scheme. In addition, parallel classification scheme of the spiral and S0 galaxies was initially suggested by van den Bergh (1976), which divides also the S0 galaxies to several classes by the prominence of their central concentration in a similar manner as spirals. Later observational studies have found support for the parallel classification (e.g. Cappellari et al. 2011; Laurikainen et al. 2010, 2011; Kormendy and Bender 2012).

1.2 Disc breaks as probes of galaxy evolution

The discs in lenticular and spiral galaxies contain substantial fraction of the total baryonic mass of stars and gas, and the angular momentum of the galaxies. In addition, the discs host most of the diﬀerent structures in galaxies. As mentioned above, many have spectacular bar and spiral structures. These structures drive the internal evolution of galaxies, by transferring angular momentum inside galaxies. Manifestation of this internal evolution are for example beautiful symmetrical ring structures seen in the disc of many galaxies. However, the environment is equally important in shaping the disc

3 structure, and galaxies in general. Properties of galaxies are commonly researched by examining their surface brightness distributions. Galaxy photometry was first done by Reynolds (1913), who studied the light distribution in the bulge of M31. The discs are much fainter structures and their study was started several decades later, as the observational instruments became more sensitive. Classical picture of a disc galaxy radial surface brightness profile consists of a bulge with a peaked surface brightness in the centre, followed by an exponentially decreasing surface brightness of the disc. The paradigm of exponentially decreasing disc light has stood, but several studies have revealed that the discs show global deviations from single exponential surface brightness distribution. These studies have shown that discs can be classified into three major categories: single exponential Type I, Type II with down-bending break of steeper decline of surface brightness outside the break, and Type III with up-bending break of less steep decline of surface brightness outside the break. The change of slope in the profiles of Types II and III are known as disc breaks. In fact, it has been established that the classical single exponential discs are found only in approximately one third of disc galaxies. The disc breaks are thus very common structures. The formation of the breaks has been attributed on a wide variety of internal and external evolutionary processes of galaxies, such as bars, galaxy interactions, and star formation. Therefore, they provide an excellent laboratory to estimate to what extent the discs in galaxies are modified by the various mechanisms. In this thesis the properties of galaxy discs are studied using surface photometry of mainly the mid-infrared 3.6 µm images from the Spitzer Survey of Stellar Structure in Galaxies (S4G). Infrared data is dominated by old stars with low masses, which also comprise most of the stellar mass in galaxies. This provides a look into the stellar mass distribution of galaxies, and a new view to the disc break phenomena, mostly studied in optical wavelengths previously. We begin in chapter 2 by describing the formation of disc galaxies, and the evolutionary processes of galaxies. In chapter 3 the different disc profile types and their proposed formation mechanisms are discussed. In chapters 4 and 5 we introduce our data, sample, and the data analysis methods. Finally, we present some of our results in chapter 6, and give a brief summary in chapter 7.

4 Chapter 2

Galaxy formation and evolution

2.1 Structure formation in the Universe

The formation and evolution of galaxies is intimately connected with the Lambda Cold Dark Matter (ΛCDM) cosmological model, in which the structure formation in the Universe can be divided into five epochs: the very early Universe, the radiation dominated Universe, the linear growth epoch, the non-linear growth epoch, and the galaxy formation epoch. In the very early Universe, events taking place after Big Bang (BB) before the Planck time is reached (10−44 seconds) cannot be studied with the laws of physics. Very shortly after, at about 10−36 seconds after BB, a period of rapid expansion started, and lasted until the Universe was approximately 10−32 seconds old. This rapid expansion, inflation, was introduced by Guth (1981) to solve the horizon and flatness problems. The former problem deals with the fact that remote regions of the Universe should not have been in causal connection, yet somehow have similar physical properties. The flatness problem describes that even small deviations in the values of matter and energy density from the critical density of the Universe would grow rapidly in time, and result in an non-flat geometry of the Universe. Both of these problems would be solved by the short inflation period, but what set off the inflation and how it ended are unknown. Direct observational evidence is also lacking, but observations of the Cosmic Microwave Background do show that the Universe is homogeneous and isotropic, and very close to flat geometry (e.g. Spergel et al. 2007). During the radiation dominated era, the properties of the Universe were set by photons and neutrinos (∼ 10−10 seconds to ∼ 50000 years after BB). The Universe continued to expand, at much reduced rate than during inflation, and the density and temperature of the Universe kept decreasing. A few minutes after BB, the temperature had fallen sufficiently for fusion of protons and neutrons to take place forming atomic nuclei. This is known as the Big Bang Nucleosynthesis, which produced deuterium, and also some

5 isotopes of helium and lithium.

Eventually the energy density of matter exceeded that of radiation. Dur- ing the short inflation period quantum fluctuations were stretched to macro- scopic scales (Guth 1981), creating small density perturbations in the matter distribution acting as the seeds for the structure of the Universe. During the linear growth epoch it is these initial perturbations which further grow linearly. The growth rate of the perturbations depends on the density contrast of the Universe (i.e. the difference between local and mean density, δρ/ρ), which increases due to self-gravity of the perturbations. When the density contrast is small the perturbations grow linearly. The expansion of the Universe continues and hydrogen and helium atoms form rapidly by capturing free electrons, as the temperature decreases and average photon energies fall below the ionization energies of hydrogen and helium. This is known as recombination. The sudden reduction of free electrons results in the decoupling of matter and photons, and allows photons to travel freely. The last scattering of photons by free electrons is observed in the Cosmic Microwave Background (CMB) radiation. After recombination the Universe entered the Dark Ages, during which Universe evolved adiabatically and was filled by the CMB photons, neutral gas, and dark matter.

The linear growth epoch ends once the density contrast reaches a value of δρ/ρ ≈ 1. The structure growth becomes highly non-linear and essentially structures start to form and collapse due to gravitational instabilities, starting first in the smallest scales in the cold dark matter paradigm. Dark matter and primordial baryonic gas create over-dense regions, halos. In turn, the baryonic gas dynamical effects become important in the evolution of the halos. During the initial collapse of the halos the dark matter is violently relaxed and the primordial gas is heated by shocks. The halos of dark and baryonic matter gain angular momentum through tidal torques from neighbouring halos (Tidal Torque Theory, e.g. Doroshkevich 1970). The formation of the halos is also intimately connected with the two-stage framework of galaxy formation suggested by White and Rees (1978): in the first stage the dark matter structures collapse into individual halos which further grow hierarchically, and in the second stage the galaxies form inside the dark matter halos from the collapsing gas (discussed in more detail in the next section). The formation of first galaxies and stars reionizes the neutral gas in the Uni- verse and ends the Dark Ages. The halos and galaxies continue to merge, and in every major merging event (i.e. the halos have comparable masses) the angular momentum distribution of the dark matter and gas is effectively redistributed, and the gas is shock heated again losing its previous structure. Eventually the numerous merging events also create the large scale structures of galaxy clusters and filaments.

6 2.2 Galaxy formation

Finally the epoch of galaxy formation starts. Discovery of dark matter in clusters (Zwicky 1933, 1937) and dark matter halos around individual galaxies through the rotation curves of galaxies (Bosma 1978; Rubin et al. 1980) put forward the above-mentioned two-stage galaxy formation theory of White and Rees (1978). Soon after this model was refined by Fall and Efstathiou (1980) to also include angular momentum of the dark matter halos, gained through tidal torques with neighbouring halos. Fall and Efstathiou further suggested that the hot gas contained in the halos initially shares the mass and velocity distribution of the dark matter. During the collapse and numerous merging events of the halos, the baryonic gas is shock heated to the virial temperature of the halo. However, in addition to gravity, the gas is also affected by dissipational processes: the gas is able to cool and collapse into dense clouds. Criteria for the collapse is that the cooling time has to be smaller than the dynamical time (e.g. White and Rees 1978, and review by Silk et al. 2013). When this requirement is fulfilled, the gas clouds begin to collapse to the potential well of the dark matter halos. During the collapse, the initial angular momentum of the gas is conserved and the gas settles into a thin rotating disc, in which the inner regions are assembled first and the disc build-up gradually moves outwards. This inside-out build-up of the gas disc is related to the angular momentum distribution of the collapsing cloud: gas with lowest angular momentum collapses first. Further fragmentation into smaller gas clouds due to Jeans instabilities can occur and set off star formation. Perturbations larger than the Jeans scale become unstable and collapse. This scale is defined as: πv R = √ s , (2.1) J 4πGρ where vs is the speed of sound in the gas, and ρ is the density. When the sound waves in the cloud are not able to cross the over-dense region in a timescale −1/2 shorter than gravitational free-fall time (tff ≈ 0.5(Gρ) for a spherical mass distribution), the pressure changes are not communicated across the cloud and it will collapse due to its own gravitation. The minimum mass of the over-dense region that can collapse can be derived from this, known as Jeans mass: 4π R 3 T 3/2 M = ρ J ∝ , (2.2) J 3 2 ρ1/2 where T is the temperature of the gas. The cooling rate of the gas is pro- portional to the collision rate of molecules in low densities (number density 4 −3 n . 10 cm , e.g. Lepp and Shull 1983), in which densities each collision leads to an excited state of the molecules and radiative cooling. Thus as the gas density increases the Jeans mass reduces. In practice this means that

7 the gas further fragments into smaller dense regions which host individual star formation, and gives place to the stellar initial mass function. The first stars formed are of population III, which are thought to be very massive and luminous due to the deficit of heavy elements in the gas they were formed from. In turn, they are short lived, and enrich the gas with heavier elements further increasing the efficiency of the gas cooling, and giving rise to the formation of less massive stars. Star formation continues more efficiently, and first galaxies are formed in the centres of dark matter halos. Bulk of the star formation will take place in the rotating gas disc. Mergers between the halos, gas, and galaxies continue, redistributing the now formed stars and reheating the gas in the halos. This constitutes the bottom-up and hierarchical growth scenario of galaxy formation, where galaxies and dark matter halos gain their mass through numerous merging events. What is then the initial structure of the formed thin stellar disc? This question was studied already before the paradigm of galaxies forming inside the dark matter halos. Several authors (e.g. Mestel 1963; Crampin and Hoyle 1964) showed that a spherical, uniformly rotating gas cloud contracts to a disc, which has a rotation curve similar to those in real galaxies. Explanation for the initial rotation came when galaxies were considered to form inside rotating dark matter halos, as explained above. Dalcanton et al. (1997) and Mo et al. (1998) expanded these early models, and have been able to reproduce properties of real galaxies, such as the flat rotation curves of discs and the Tully-Fisher relation (Tully and Fisher 1977), connecting galaxy mass or luminosity with the galaxy rotational velocity. Observations have shown that discs have roughly exponentially decreasing radial surface brightness profiles (e.g. Patterson 1940; de Vaucouleurs 1959b; Freeman 1970). This exponential light profile is commonly described with the exponential function: r I = I exp − , (2.3) r ◦ h where I◦ is the disc surface brightness in the centre of the galaxy, h is the scalelength of the disc, and r is the radial distance from the galaxy centre. A satisfactory explanation for the formation of exponential stellar discs has not been found, although two possible mechanisms to produce initially exponential discs have been proposed. First, it is possible that an exponential gas disc is formed, when the angular momentum is conserved during the gas collapse inside the dark matter halo. This gas disc later on hosts star formation and the formed stellar disc will retain the same initial distribution (Fall and Efstathiou 1980). Alternatively, viscous torques could redistribute the gas in the disc towards exponential distribution from an arbitrary initial distribution. Essentially the gas distribution would be modified by angular momentum transport, resulting in an exponential stellar disc if the star formation timescale is similar to the viscous timescale (Lin and Pringle 1987).

8 Both of these mechanisms proposed to form exponential discs have problems explaining the lack of low specific angular momentum material of real galaxies: the models form galaxies that are too centrally concentrated. Usual explanations given for the model shortcomings are their use of unrealistic initial conditions or incomplete inclusion of baryon physics, such as star formation and different feedback mechanisms. Indeed, the complex gravitational and gas hydrodynamical processes are not well understood during disc formation. For example, it has been argued that inclusion of feedback processes might not be enough to produce completely exponential discs (e.g. Dutton 2009, see section 2.3). Observations of high redshift, moderate to high mass galaxies (z > 1, 10 M∗ > 10 M ), have revealed that they often have very clumpy appearance (van den Bergh et al. 1996), while some galaxies also show disc-like rotation (e.g. Genzel et al. 2006). Models suggest that the initially collapsed gas discs are turbulent and quickly fragment into massive star forming gas 9 10 clumps (even 10 M in galaxies with total stellar mass M∗ > 10 M ), which migrate inwards forming part of the central bulge (Noguchi 1999). The clumps are affected by dynamical friction and gravitational torques of the stellar and gaseous disc, as well as by the dark matter halo. As a result, the clumps lose angular momentum and migrate inwards, while the disc and the halo are heated (Bournaud 2016). The migration timescale is short, < 500 Myr, and depends on the initial distance where the clump is formed (Bournaud et al. 2007). However, these clumps can be destroyed by feedback processes before reaching the galaxy centre, if the masses of the clumps 7 are sufficiently low (. 10 M , e.g. Perret et al. 2014). The formation and migration of the massive clumps towards the galaxy centre can also affect the properties of the stellar disc. Indeed, simulations show that the clumps stir the already existing stellar disc, causing exponential disc profile to be gradually formed regardless of the initial structure (Bournaud et al. 2007; Elmegreen and Struck 2013). These results suggest that the discs are not necessarily initially exponential, but this shape is achieved later through internal evolution. However, the precise formation mechanisms of the discs are not well understood.

2.3 Issues of galaxy formation models

The structure and galaxy formation is studied by using numerical semi- analytical models, and N-body cosmological simulations. Large scale N-body simulations have successfully reproduced the large scale structure of ﬁlaments and voids, by using cosmological parameters derived from observations. Ex- amples of such simulations include the Millenium-I and Millenium-II simulations (Springel et al. 2005b and Boylan-Kolchin et al. 2009, respectively),

9 NGC5457 IC5249 600

100 400

50 200

0 0

-200 -50

-400 -100

-600 -600 -400 -200 0 200 400 600 -100 -50 0 50 100

Figure 2.1: Two examples of galaxies dominated by the disc structure, seen face-on (left) and edge-on (right). The face-on galaxy NGC 5457 does not have a classical bulge, but instead has a small discy pseudo-bulge in the centre. The edge-on galaxy IC 5249 does not show signs of classical bulge component, which would extend above the disc plane. The images are 3.6 µm images from the Spitzer Survey of Stellar Structure in Galaxies (S4G, Sheth et al. 2010), with the surface brightness levels 14-27 AB mag arcsec−2 shown. The scales of x- and y-axis are in arcseconds. which consider only the dark matter component. The baryonic physics have been commonly included in semi-analytical models of galaxy formation, but today the most advanced simulations track the evolution of both the dark matter and baryonic components together. However, the baryonic physics are included using simpliﬁed recipes (e.g. ILLUSTRIS (Vogelsberger et al. 2014) and EAGLE (Crain et al. 2015) simulations). Despite the increasing sophistication and ability to reproduce properties of the observed Universe, the simulations and semi-analytical models have common problems. Galaxies forming in simulations and semi-analytical models tend to have too large central concentrations compared to real galaxies. The problem is especially pressing as galaxies which seem to have no classical central bulge component (e.g. 19 Sc-Scd galaxies analysed by Kormendy et al. 2010), or no bulge component at all (e.g. Sachdeva and Saha 2016), are common in the Universe (examples shown in Fig. 2.1). In paper III a sharp drop in the bulge fraction is found also for galaxies above T ∼ 5 (see Fig. 33 in paper III). The problem can be even more acute than previously assumed since many bulges even in the S0s interpreted to be classical merger-built bulges, can in fact be bar-related phenomena (Laurikainen et al. 2014; Athanassoula et al. 2015). In simulations the cooling of the baryonic gas is very eﬃcient, and commonly the gas has already cooled when it reaches the galaxy forming

10 in the centre of the halo. The halos further go through hierarchical merging processes, after which the already cooled baryons fall to the centre of the potential well, losing their angular momentum through dynamical friction. The central bulge components in such simulations are too large, and also the discs are too centrally concentrated (e.g. Navarro and Benz 1991; Navarro and White 1994; Steinmetz and Navarro 1999, and review on the context of bulges by Brooks and Christensen 2016). This is known as the angular momentum or overcooling problem. Suggested solutions for these problems include feedback processes from young stars and supernovae (Stinson et al. 2012; Aumer et al. 2013; Roˇskar et al. 2014), or active galactic nuclei (AGN, e.g. Springel et al. 2005a; De- Buhr et al. 2010). It is argued that the role of active galactic nuclei is minor. Instead, the increased feedback from supernovae after a burst of star formation redistributes the gas in the disc, and possibly ejects a fraction out of the disc, and restricts the growth of the central concentration (e.g. Governato et al. 2010; Brook et al. 2011 and Brook et al. 2012, respectively). The increased star formation could be triggered by, for example, a merging event. However, a comprehensive solution has not been found yet. The properties of the dark matter halos retrieved from simulations do not fully follow the results of observations. Large cosmological simulations indicate that the dark matter halo density profiles can be described by a power law, regardless of the mass of the halo (i.e. cuspy profiles). However, observations suggest that low surface brightness galaxies and late-type gas rich dwarf galaxies have a density profile with a much flatter core (i.e. core profile; see the review by de Blok 2010 and references therein). Furthermore, observations of dwarf galaxies by Lelli et al. (2014) and of moderate mass spiral galaxies by Erroz-Ferrer et al. (2016) have shown that the inner rotation curves of galaxies are steeper for higher central concentration of luminous matter, indicating that galaxy centres are baryon dominated. Simulations of single galaxies show that supernova feedback removing low angular momentum gas could be altering the central regions of the dark matter density profiles to flat core profiles in late-type dwarf galaxies (Governato et al. 2010). Whether similar processes could work in more massive low surface brightness galaxies is uncertain (Kuzio de Naray and Spekkens 2011). In the current cosmological framework dark matter halos are assumed to gain mass through hierarchical clustering, starting from the smallest scales. In simulations this results in an over-abundance of low mass halos, when compared to the observed frequency of low mass dwarf galaxies (Klypin et al. 1999; Moore et al. 1999a). This is known as the missing satellite problem. Nevertheless, the lowest mass galaxies have longer star formation time-scales (e.g. Hunter and Gallagher 1985; van Zee 2001), and in principle it is possible that the lowest mass satellite galaxies might not have formed significant amount of stars during their lifetime, and are too diffuse to be observed.

11 2.4 Galaxy evolution

After the relatively fast initial assembly of halos and discs, the evolution of galaxies proceeds in a slower pace. There are no clear cut phases, and the build-up of galaxies continues still via gas accretion and internal dynamical evolution. The visible component of galaxies, stars, are embedded in a dark matter halo, and especially in spiral galaxies the stellar disc is complemented by an extended gas disc. The evolutionary processes that aﬀect all these components can be further divided into external and internal by their origin. Relative importance of individual processes on the overall evolution of galaxies is diﬃcult to estimate, although this is a major aspect of current galaxy evolution studies.

2.4.1 External evolution It was noted by Hubble and Humason (1931) that early-type galaxies are preferentially located in rich galaxy clusters, compared to spiral galaxies which appear to be more isolated. This observation of the importance of environment for the galaxy morphology was quantiﬁed in the morphology- density relation introduced by Dressler (1980). The relation shows that in most dense environments, galaxy cluster centres, the fraction of elliptical and S0 galaxies can be as high as 90%. Meanwhile in the ﬁeld environments the fraction of spirals is ∼ 80%. These observations indicate that galaxy evolution and morphology is intimately connected to the interaction with its environment.

Galaxy mergers Interactions between dark matter halos and galaxies form the basic paradigm of the hierarchical structure growth in the Universe, and galaxies are expected to go through multiple mergers during their life-time. The merging events of galaxies are categorized by the mass ratio of the involved objects to minor (M/m & 4) and major (M/m ≈ 1 − 4) mergers. The frequency of galaxy mergers was higher in the early Universe, and the peak time of merging events also depends of the galaxy mass. Observational results of Conselice et al. 10 (2008) show that galaxies with mass larger than 10 M have experienced roughly four major merging events since z ∼ 3, with most of the merging 8 9 events occurring before z = 1. For low mass galaxies of 10 − 10 M the number of major mergers since z ∼ 3 is roughly two, and they have reached the peak of merging rate at lower redshift. Not all close encounters of galaxies, and their dark matter halos, lead to merging. The relative velocity of the encounter has to be low enough, comparable to the internal velocities of the galaxies, for a merging to occur

12 (e.g. Binney and Tremaine 2008). Thus mergers occur mainly in groups of galaxies while they are rare in dense galaxy clusters, in which the velocity dispersion of galaxies is too high. Instead, in clusters the galaxy encounters occur as high speed passings, usually called galaxy harassment (discussed in more detail below). Galaxies also interact through the extended dark matter halos in which they reside. The halos experience a drag force even before signs of visual interaction between stellar discs are evident. Due to this drag, the dynamical friction, the galaxies convert energy from their relative motion to internal random motions, which causes the orbits of the galaxies to decay and result in the actual merging. The effectiveness of dynamical friction depends on the density of the halos, and the mass and velocity of the satellite. Denser halos are more effective in absorbing the energy of the satellite, as are encounters in moderate velocities. Lower mass of the satellite galaxy also reduces the dynamical friction, and increases the merging timescale (e.g Boylan-Kolchin et al. 2008). Especially the growth of the cluster centre cD galaxies is thought to be due to the dynamical friction of the extended cluster halos, which eventually causes the massive satellite galaxies to merge with the central galaxy. This process is known as galactic cannibalism. The end result of a galaxy merger depends strongly of the mass ratio of the galaxies. During the merging, the orbital energy of the galaxies is transferred to the internal energy of the remnant and of the dark matter halo. In major mergers the stellar structures of the progenitor galaxies are completely destroyed, while the gas is shock heated. The early simulations of Toomre (1977) already suggested that major mergers of disc galaxies form elliptical galaxies (see also Barnes 1992; Mihos and Hernquist 1996). Toomre and Toomre (1972) and Toomre (1977) also showed that spectacular tidal tails can form as a result of the merging, depending on the orbital properties of the encounter, as seen in the Mice galaxies (Fig. 2.2). Further evidence of merger origin of elliptical galaxies comes from the observations of faint loops and shells around many of these galaxies, which also form in merging events (e.g. Hernquist and Spergel 1992). It is also possible that during the scrambling of stars in a major merging a classical bulge, and a vertically extended thick disc structures are formed. Furthermore, the gas shock heated during the merging, could cool and accrete back from the halo, forming a new gaseous and stellar disc (Springel and Hernquist 2005; Athanassoula et al. 2016). In minor merging events the more massive progenitor galaxy might be only slightly disturbed in appearance. The small satellite galaxy loses orbital energy, which will be absorbed by the more massive galaxy and the dark matter halo. This results in heating of these systems. If the massive progenitor is a disc galaxy, the absorbed energy will dynamically heat the disc in vertical and radial directions: the disc will spread radially and flare in vertical direction. The most noticeable changes will occur in the outskirts

13 Figure 2.2: NGC 4676, or The Mice, showing tidal tails with strong star formation in the ongoing major merger of two galaxies. Image credit: NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA. of the galaxy, where the surface density is low. Depending on the orbit of the satellite galaxy the outer parts of the disc can warp (e.g. Velazquez and White 1999), and the encounter could also trigger instabilities in the disc which result in the formation of a bar structure (e.g. Noguchi 1987; Gerin et al. 1990; Salo 1991). Furthermore, the dense core of a small satellite galaxy could survive the merging, and sink to the centre of the more massive galaxy. This is one possibility for the formation of a classical, velocity dispersion supported bulge (Tremaine et al. 1975; Aguerri et al. 2001). If the more massive galaxy is instead an elliptical galaxy, the merging with a small disc galaxy can result in the formation of complex shell structures around the galaxy (e.g. Quinn 1984), similarly to major merging events.

14 The amount of gas in the progenitor galaxies is also important for the out- come of merging. In the presence of cold gas (“wet” mergers) the interactions results in enhanced star formation. The gas itself also dissipates the energies of the merging by losing and gaining angular momentum, and experiencing feedback effects. Wet minor merger events involving a disc galaxy will also cause the gas to be driven to the centre of the disc, driving a central star burst event. On the other hand, the most loosely bound gas in an extended gaseous disc could be flung away, forming long tidal tails. The merging of two gas poor elliptical systems (“dry” merger) was already examined in the simulations of Toomre (1977). This kind of merging with two roughly equal mass elliptical progenitors, can make the most massive elliptical galaxies, which are systems with small rotation, and boxy elliptical isophotal shapes with flat centres in the surface brightness (e.g. Naab and Burkert 2003; Naab et al. 2006; Emsellem et al. 2011). Cox et al. (2006) studied merging of equal mass disc galaxies, with gas effects included in their simulations, and produced elliptical galaxies with rounder and more compact cores, compared to dry mergers. The inflow of gas triggers star formation in the centre of the remnant, causing feedback driven gas removal from the galaxy, resulting in elliptical galaxies with rounder and denser cores. These results agree well with the observed properties of intermediate and low mass elliptical systems, and suggest that elliptical galaxies could be formed through two separate merging channels. The initial conditions of the merging process are especially important when disc galaxies are involved, in particular the orientation of the disc planes with respect to the orbital plane of merging. The first simulations of Toomre and Toomre (1972) showed that the prominence of tidal tails formed through merging depends on the morphology of the progenitors, and the orbit of the encounter. They found that the tails are narrow due to their origin from dynamically cold discs. Mergers between dynamically hot elliptical galaxies produce shells and loops as described above, instead of tidal tails. In addition, Toomre and Toomre noted that when the orbit of the merging is aligned with the spin of the discs (i.e. a prograde orbit) the produced tidal tails are more prominent, due to the orbital angular momentum being coupled to the rotation of the stars in the disc. Similarly, mergers with orbital inclinations close to the plane of the disc are more efficient in transferring angular momentum to the stars in the disc (e.g. Younger et al. 2007).

Galaxy harassment

Galaxy harassment is a cumulative eﬀect of many high-speed encounters between galaxies. It is mostly taking place in rich galaxy clusters, where galaxies go through constantly varying gravitational potential ﬁeld. Sim- ulations have shown that the continuing harassment can alter the galaxies

15 significantly. In addition, it is most efficient for late-type Sc-Sd spiral galaxies, which have fragile extended thin discs with low surface densities (e.g. Moore et al. 1999b). The multiple encounters cause formation of tidal tails due to impulsive heating of the galaxy, which increases its internal energy and moves stars to unbound orbits (Moore et al. 1996, 1998). This further causes morphological transformation of the harassed disc galaxy to an object resembling dwarf elliptical or spheroidal galaxy (Kormendy and Bender 2012). As much as 25-75 % of the stars of the progenitor galaxy could be stripped out by the harassment during a 5 Gyr period, and the discs can be completely destroyed by one or two cluster centre passages (Farouki and Shapiro 1981; Moore et al. 1998). The late-type spiral galaxies also have large amounts of gas in their disc. The impact of the angular momentum loss during the harassment can cause the gas to inflow to the centre of the galaxy, enhancing star formation and fueling of an active galactic nuclei.

Ram-pressure stripping Galaxy clusters are filled with hot intracluster gas that has temperatures up to 107 − 108 Kelvins. When passing through this medium, the internal gas of the halo and disc can be swept away by the pressure of the intracluster gas, as first suggested by Gunn and Gott (1972). The least bound gas in the halo and galaxy outskirts are removed first, and the removal continues from outside in. Observational support for this process has been found in the Virgo and Coma clusters, where spiral galaxies closer to the cluster centre have distorted and smaller HI gas discs, when compared to galaxies in the cluster outskirts (e.g. Cayatte et al. 1990; Chung et al. 2009). The gas stripping has also been observed in action. In the Virgo cluster Chung et al. (2007) found galaxies at projected distances less than 1 Mpc from giant elliptical galaxy M87, which show HI tails pointing away from M87. Kenney et al. (2004) have observed HI distribution in the Virgo cluster spiral galaxy NGC 4522, which is often used as a textbook example of ram-pressure stripping in action. This particular galaxy has an unperturbed stellar disc, but the HI gas distribution is trailing and sharply cut off. When the gas is removed from spiral galaxies during their journey deeper into a cluster, their potential for forming stars is also sharply reduced. Ram- pressure stripping, working together with galaxy harassment, has been suggested to transform spiral and irregular galaxies to S0 and spheroidal galaxies (e.g. Kormendy and Bender 2012), which would provide at least partial explanation why more of S0 and spheroidal galaxies are observed in dense environments. However, the gas removal is thought to be insufficient for the formation of the S0s with the most massive bulges, and additional processes

16 are needed to explain the bulge growth (see discussion in Laurikainen et al. 2013).

Galaxy strangulation While ram-pressure stripping is able to remove all the gas from a galaxy, it might not be the case for galaxies experiencing strangulation. Instead, only the most loosely bound hot diffuse gas in the galaxy halo might be removed. This would result in a gradual reduction of star formation, as the remaining gas in the disc is used, and no reservoir is left in the halo from which to further fuel star formation. The strangulation process was first proposed by Larson et al. (1980) as a possible pathway to form S0 galaxies from spirals. Observations show that galaxies with similar masses and internal structures have star formation rates dependent on the environment: the galaxies in dense environments form stars in a reduced rate (Kauffmann et al. 2004). It has been claimed that this dependency is mainly controlled by the galaxy strangulation process (e.g. Balogh et al. 2000; van den Bosch et al. 2008).

Gas inflows The above mentioned environmental processes primarily remove gas from galaxies, and thus reduce star formation over time. However, galaxies and galaxy clusters reside in complex filaments filled with gas, which gas can cool down and accrete into galaxies. This gas would settle into the outskirts of the galaxies, where it could increase the normally very low star formation rates. Indeed, extended discs in the ultraviolet light have been observed around approximately ∼ 25 % of disc galaxies, revealing star formation taking place much further out than the optical stellar disc (Gil de Paz et al. 2005; Thilker et al. 2005; Zaritsky and Christlein 2007; Lemonias et al. 2011). The origin for this enhanced star formation at the outskirts of galaxies has been suggested to be due to inflow of cold gas (e.g. Lemonias et al. 2011; Moffett et al. 2012; Moran et al. 2012). The accreted gas could also disturb the existing stellar disc, causing it to extend radially, as shown in the simulations of Minchev et al. (2012).

2.4.2 Internal evolution The internal evolution timescales of galaxies are generally longer than those of environmental processes. The major mode of evolution has changed from environmentally to internally driven in low redshift, as galaxy mergers become less frequent. Rotating thin galactic discs are also inherently unstable. These instabilities enable the formation of non-axisymmetric structures, which drive angular momentum transport and internal evolution inside galaxy discs. These instabilities can be divided into global and local. The

17 former cause significant changes in the overall structure of the disc, while the latter control whether small perturbations in the disc can grow, such as the formation of star forming molecular clouds. Star formation is intimately connected with the internal evolution of galaxies. Star formation is controlled by several different processes, which regulate the local and global gas distribution in galaxies. These processes include the angular momentum transport by non-axisymmetric structures, interactions with the galaxy environment, and different feedback processes such as supernovae and stellar winds. To a first approximation, the local gas density is a fundamental constraint, due to low densities being stable against gravitational collapse. This is implied by the Jeans instability criteria (Eq. 2.2) as well as by the empirical Kennicutt-Schmidt law, which connects the local gas surface density to the local surface density of star formation rate (Kennicutt 1998). With increasing radius from galaxy centre the density of the gas disc reduces, manifested also in the low star formation rates at the outskirts of galaxies. However, even in low densities star formation can be triggered by various mechanisms. Gas compression by turbulence or supernovae can disturb the normally low density gas and cause star formation (e.g. Mac Low and Klessen 2004), and as mentioned above in section 2.4.1, gas inflow from the intergalactic medium can trigger star formation at a large radius.

Bars

Bars are elongated structures in the centres of galaxies, which can range from very obvious structures to small and weak, as demonstrated in Fig. 2.3. Oval structures are also seen in galaxies, and while these are not classified as bars they do have similar dynamical effects. Bars are very common in disc galaxies, and approximately two thirds of disc galaxies at low redshift are barred (e.g. Eskridge et al. 2000; Whyte et al. 2002; Laurikainen et al. 2004; Menéndez-Delmestre et al. 2007; Marinova and Jogee 2007). The exact fraction of barred galaxies depends on the definition of bars included in the study (i.e. “strong” or “weak” bars), the used detection method, and the morphological types of sample galaxies. Studies have shown that the fraction of bars is lower in S0 galaxies (∼ 40%, e.g. Laurikainen et al. 2013; D´ıaz- Garc´ıaet al. 2016), and increases for the low mass late-type spiral galaxies (∼ 80%, T > 5, Buta et al. 2015; D´ıaz-Garc´ıaet al. 2016). Bars appear slightly stronger in the infrared bands (Eskridge et al. 2000), in which dust extinction is lower compared to optical wavelengths especially in the central regions of galaxies. In addition, bars are composed mainly of older stars, whose light is more dominant in the infrared wavelengths. Even in the first simulations of disc galaxies bars formed spontaneously through the internal evolution of the disc (e.g. Hohl 1971; Ostriker and

18 Figure 2.3: Examples of galaxies with diﬀerent bar and spiral morphologies. Shown are 3.6 µm images from the Spitzer Survey of Stellar Structure in Galaxies (S4G, Sheth et al. 2010), with the surface brightness levels of 14- 27 AB mag arcsec−2 shown. The bars are highlighted with red ellipses, and outer rings with dashed green rings, using measurements from Herrera- Endoqui et al. (2015). The mid-IR classiﬁcation is taken from Buta et al. (2015). The scales of x- and y-axis are in arcseconds.

19 Peebles 1973). Furthermore, bars tend to form naturally in simulations if the velocity dispersion of the disc stars is not very high. This can be expressed using the Toomre’s local stability criterion (Toomre 1964) for a stellar disc: σ κ Q ≡ R , (2.4) 3.36GΣ where σR is the radial velocity dispersion, κ is the epicyclic frequency, G is the gravitational constant, and Σ is the surface density of the disc. Formally, Q ≥ 1 guarantees stability against axisymmetric perturbations, but the system can still be prone to non-axisymmetric perturbations, most notably the m = 2 bar mode. In practice, when Q & 2 the disc is stable against the formation of a bar. The global dynamical instability of the disc leading to the formation of a bar, consequently called as a bar instability, moves stars to elongated orbits forming the main body of the bar (extensive review on dynamical aspects of bar formation is given in Binney and Tremaine 2008). The bar rotates like a solid body about the vertical axis of the galaxy with angular rate Ωp, i.e. the pattern speed of the bar. Intimately connected with the bar’s gravitational potential are resonances, which are the main mechanism by which the bar transfers angular momentum inside a galaxy. Perhaps the most important resonances for the evolution of the disc are the corotation resonance (CR), the inner- and outer Lindblad resonances (ILR and OLR, respectively for m = 2 azimuthal component of the bar), and the inner and outer ultraharmonic resonances (IHR and OHR, respectively for m = 4 azimuthal component of the bar). These are deﬁned in the epicyclic approximation by the relation between the bar pattern speed (Ωp), the circular angular speed (Ω), and the epicyclic frequency (κ). The CR is located at the distance where Ωp = Ω(r). (2.5) The two Lindblad resonances where κ(r) Ω = Ω(r) ± , (2.6) p 2 in which OLR has +, and ILR - sign. Lastly, the ultraharmonic resonances are located where κ(r) Ω = Ω(r) ± , (2.7) p 4 in which OHR is indicated by +, and IHR by - sign. Depending on the rotation curve, the bar may have one, two, or zero ILR’s. The main eﬀect of the bar on stars in the disc is the transfer of angular momentum, which is emitted from the inner parts of the galaxy to the disc and halo beyond the CR radius. In this process the disc extends radially, and thus also the scalelength (h) increases by a factor of 1.2-1.5, as demonstrated for example by the simulations of Valenzuela and Klypin (2003). The

20 angular momentum redistribution depends on the velocity dispersion of the disc. In the discs with large velocity dispersion the resonances are not as efficient in emitting or absorbing angular momentum, and the transfer of angular momentum is also limited. Consequently the evolution of the bar itself depends on the angular momentum transfer, but in addition to the disc, the dark matter halo is also able to significantly absorb angular momentum (Athanassoula 2002). The bar rotation will slow down as more angular momentum is moved to other components, which will also increase the length of the bar and move the resonances further out in the galaxy. The response of the gas to the bar differs significantly from the stars. This is due to the dissipative properties of gas, which become evident when two gas clouds collide or pass near each other at the resonances, where periodic orbits cross. Essentially this could lead to the formation of gaseous rings at these resonances. Accumulation of gas also promotes enhanced star formation, and eventually the rings also manifest in the distribution of stars. This process has been shown in many simulations (e.g. Salo et al. 1999) to result in the formation of outer ring structures at the OLR, or in some cases at the OHR. Inner rings are located near the IHR, and nuclear rings at the ILR. Outer, inner, and nuclear rings are observed in large number of galaxies. The fraction of galaxies with inner and/or outer rings peaks near Hubble type T = 0, where even 40 % of galaxies have outer rings and 60 % have inner rings (Buta and Combes 1996; Comerónet al. 2014). The gas inflow towards the centre of a galaxy can also result in the formation of a discy pseudo-bulge structure, and fueling of central star formation (conclusive reviews by Fisher and Drory 2016; Kormendy 2016).

Spiral structure

Spiral structures in galaxies are sites of spectacular star formation, and their appearance is also one of the backbones of Hubble’s original galaxy classification. In addition, the spirals are generally divided into the grand-design and flocculent spirals. Grand-design spirals are commonly found in barred galaxies, and are quite symmetric structures which can be traced continu- ously from the beginning to the end. On the other hand, flocculent spirals do not have a well defined structure and usually consist from many short sections. Cold gas and star formation in the disc are fundamental ingredi- ents that seem to be needed for the presence of a spiral structure, due to the striking lack of spiral structure in S0 galaxies, which have very little cold gas. Indeed, the spiral structures are clearest in the blue wavelengths, where the star formation clusters and young stars along the spiral arms are pronounced. The grand-design spirals are also prominent in the infrared wavelengths, contrary to the flocculent spirals which are less evident in infrared wavelengths (Elmegreen and Elmegreen 1984). The dominant source of light in infrared

21 are old and cool stars, which constitute most of the stellar mass of galaxies. Thus especially the grand design spirals are not only the locations of increased star formation, but they are also present in the mass distribution of the galaxies. The formation of spirals is not fully understood. The presence of the spiral arms in the mass distribution confirms that they are places of enhanced density. Indeed, Lin and Shu (1964) introduced the quasi-stationary density wave theory for the grand-design spiral arm propagation and growth, which states that spiral structure is a long-lived, rigidly rotating density enhance- ment, formed due to one of the fundamental instabilities in a galactic disc. The instability would grow over time and saturate to a certain non-linear level resulting in persistent spiral structure. Bars could trigger the spiral formation (e.g. Kormendy and Norman 1979; Salo et al. 2010), as could also interactions between galaxies as shown first in the simulations of Toomre and Toomre (1972). The close passage of a satellite galaxy during minor merging event could trigger the formation of a grand design spiral structure, and the following passages would maintain the spiral structure (Salo and Lau- rikainen 2000a,b). In these well defined grand-design spiral structures the gas is shocked and compressed as it enters the spiral density wave, resulting in star formation. Alternatives for the density wave theory have also been presented. Floc- culent spirals could simply arise from the differential rotation of the galaxy discs. Star formation born out of local instability in a small region of the disc would be stretched kinematically to short spiral segments, as discussed for example in Binney and Tremaine (2008). Hence it is thought that the grand-design spirals are born out of global instabilities of the disc, and are relatively long-lived although constantly moving structures, while the flocculent spiral arm segments are more short-lived phenomena caused by local disc instabilities. Dynamically the spiral structure increases random motions of stars and enables radial excursions of stars (see review by Sellwood 2014). Angular momentum of individual stars can be either increased or decreased, and thus the spiral structure does not greatly alter the global angular momentum distribution (Sellwood and Binney 2002). Increased random motions of stars causes radial mixing in the discs. An effect of this kind has been observed also in the Solar neighbourhood, where older stars have larger velocity disper- sions (Wielen 1977; Nordströmet al. 2004). A major factor for this heating is scattering by Giant Molecular Clouds along the spiral arms (Ida et al. 1993), with contributions also from the Lindblad resonances of the spiral mode (Sellwood and Binney 2002). Stars also experience substantial changes in their orbits in a manner which does not heat the disc. This is commonly known as radial migration. Sellwood and Binney (2002) were the first to report of this process, in which transient spiral mode causes angular momen-

22 tum changes of stars at the spiral mode corotation. Eﬀectively, the guiding centre of the stars orbit is changed, and the star embarks on a random walk in the disc. Radial migration could also be induced by overlapping bar and spiral resonances as shown by Minchev and Famaey (2010), with the maximum changes in the angular momentum of stars taking place near the bar OLR radius. Evidence of radial migration is found in the Solar neighbourhood, where the metallicity of some stars suggests that they were formed elsewhere in the discs (e.g. Haywood 2008). Migration is a slow process, and simulations suggest that the mean stellar age increases in the outer parts of galaxy discs due to migrated old stars (Roˇskar et al. 2008b,a).

23 24 Chapter 3

Diﬀerent types of galaxy discs

3.1 Surface brightness proﬁles

The structures of galaxy discs are commonly studied using their radial surface brightness profiles. The simplest method to obtain a radial surface brightness profile of a galaxy is to take a cut in a narrow slit along the apparent major or minor axis of the galaxy, from which the surface brightness profile is constructed. An example of this is shown in Fig. 3.1, upper panels, for galaxy NGC 3184. An immediate downside of this method is that most of the image, and thus information of the galaxy, is disregarded. Thus the profile might not represent the overall behaviour of the galaxy. In addition, the signal to noise ratio is lower than with using methods which take the whole galaxy into account, especially at the disc outskirts. In edge-on galaxies these problems are avoided, and this method is commonly used to derive the surface brightness profiles of the thin and the thick discs of edge-on galaxies. For face-on galaxies the surface brightness profile is often obtained in terms of fitting elliptical isophotes, over which the surface brightness is averaged and presented as a function of ellipse semi-major axis (Fig. 3.1 middle panel). The usual work flow is similar to that described in papers I, II, and IV: the centre of the galaxy is determined, followed by fitting the background subtracted image with free elliptical isophotes. A common tool for constructing the elliptical isophotes is the ellipse task (Jedrzejewski 1987), provided in the IRAF software system (Tody 1993). When the position angle and ellipticity are allowed to vary, the surface brightness distribution follows individual structures of the galaxy, which is not always desired. Especially in studies of discs a second method of measurement is often used, with the ellipticity and position angle of the isophotes fixed to the outer disc values (papers I and IV, also for example Pohlen and Trujillo 2006; Erwin et al. 2008a; Gutiérrezet al. 2011). This is mathe- matically equivalent to deriving the surface brightness profile over circular apertures from a galaxy image deprojected to face-on orientation. Compared

25 Cut along major axis

300 ] 16 -2 18 20 200 22 24 100 26 [AB mag arcsec

µ 28 0 50 100 150 200 250 300 Semi-major axis [arcsec] 0 Cut along minor axis

] 16 -2 -100 18 20 -200 22 24 26 [AB mag arcsec

-300 µ 28 -300 -200 -100 0 100 200 300 0 50 100 150 200 250 300 Semi-major axis [arcsec] Free elliptical isophotes 300 16

200 18 ]

100 -2 20

0 22

-100 [AB mag arcsec 24 µ

-200 26

-300 28 -300 -200 -100 0 100 200 300 0 50 100 150 200 250 300 Semi-major axis [arcsec] Fixed elliptical isophotes 300 16

200 18 ]

100 -2 20

0 22

-100 [AB mag arcsec 24 µ

-200 26

-300 28 -300 -200 -100 0 100 200 300 0 50 100 150 200 250 300 Semi-major axis [arcsec]

Figure 3.1: Examples of the three primary methods of deriving the surface brightness profile of a galaxy. The galaxy shown here is NGC 3184, with the 3.6 µm image from S4G used. The overall shape is similar in all the different methods, but the individual structures stand out in a different manner. The grey area under each profile shows the behaviour of the surface brightness profile when the background sky level is over- or underestimated by the standard deviation of the background sky.

26 to the elliptical isophote fit with free position angle and ellipticity, this results in smoother radial surface brightness profiles (see Fig. 3.1 lower panels). The fixed ellipse fits are not as sensitive to non-axisymmetric structures in the galaxies, for example bars, and trace the overall structure of the underlying disc (see for example Erwin et al. 2008a). The signal to noise ratio is ultimately the limiting factor on how low surface brightness levels the radial profile can be reliably traced. As shown in Fig. 3.1 with grey area, over- or underestimating the sky brightness by the standard deviation of the sky value, significantly affects the radial surface brightness profile at a large radius. To increase the signal to noise ratio in the low surface brightness regions, the isophote spacing is usually increased with radius when constructing the surface brightness profiles over elliptical isophotes (see Fig. 3.1).

3.2 Exponential discs

The notion of exponentially decreasing surface brightness in discs dates back to the study of Patterson (1940), who for the first time used an empirical exponential function to describe the behaviour of the disc light. His study was followed by de Vaucouleurs (1959b) and Freeman (1970), which both further established the paradigm of exponential discs. Three randomly chosen examples of galaxies with exponential discs are shown in Fig. 3.2, in which it can be seen that the surface brightness in the disc decreases in a linear fashion in the magnitude scale, as far as the surface brightness profile can be traced. The exponential behaviour has also been confirmed with other methods for a number of galaxies, for example in NGC 300 with star count measurements (Bland-Hawthorn et al. 2005). As mentioned in section 2.2, it is not completely clear how the exponential disc structure has formed, but it is assumed that it is an imprint of the galaxy formation processes. Multitude of studies have examined the exponential behaviour and have established that the disc scalelengths (see Eq. 2.3 or chapter 5) are independent of the morphological type (excluding very late-type galaxies), but rather increase with galaxy mass. The scalelength depends on the observed wavelength, increasing to lower wavelengths (e.g de Jong 1996; MacArthur et al. 2003; Fathi et al. 2010; paper IV and section 6.4). Furthermore, the disc scalelength increases from redshift z ∼ 5 to present day galaxies even by a factor of 8 (Fathi et al. 2012), when considering galaxies with the same absolute magnitudes. The wavelength dependency and evolution with redshift both indicate that the disc profile is changing over time, driven by the internal and external processes. In the past 15 years the paradigm of single exponential discs has changed dramatically. In a series of papers, Pohlen and Trujillo (2006), Erwin et al.

27 NGC3938 - log10 ( M* / MO • ) = 10.53 NGC3938 16 200

150 18

100 ] -2 20 50

0 22

-50 [AB mag arcsec 24 3.6

-100 µ

-150 26 -200 28 -200 -150 -100 -50 0 50 100 150 200 0 50 100 150 200 Semi-major axis [arcsec]

IC2627 - log10 ( M* / MO • ) = 10.06 IC2627 16 100

50 ] -2 20

0 22

[AB mag arcsec 24 3.6

-50 µ

-100 28 -100 -50 0 50 100 0 20 40 60 80 100 Semi-major axis [arcsec]

NGC2604 - log10 ( M* / MO • ) = 9.93 NGC2604 16

18 50 ] -2 20

0 22

[AB mag arcsec 24 3.6 µ -50 26

Bar radius 28 -50 0 50 0 20 40 60 80 Semi-major axis [arcsec]

Figure 3.2: Three examples of single exponential, or Type I, disc galaxies seen face-on. The images are 3.6 µm images from S4G survey, with the surface brightness levels 16-27 AB mag arcsec−2 shown. The image scales of x- and y-axis are in arcseconds. The grey area under each profile shows the behaviour of the surface brightness profile when the background sky level is over- or underestimated by the standard deviation of the background sky. Bar radius from the measurements of Herrera-Endoqui et al. (2015) is marked with vertical blue dashed line in the surface brightness profiles.

28 (2008a), and Guti´errezet al. (2011) (hereafter P06, E08, and G11, respectively) studied the discs proﬁles, using optical data of high mass moderately inclined galaxies across a large range of Hubble types (−3 . T . 8). Collec- tively they found that single exponential discs are present in only 21 ± 3% of their sample galaxies. Majority of the galaxies have more complex disc structure, which are discussed further in sections 3.3 and 3.4.

3.3 Down-bending disc breaks

3.3.1 Edge-on versus face-on In his pioneering study Freeman (1970) showed that face-on galaxy discs can be divided into two categories: Type I with completely single exponential behaviour outside the bulge, and Type II which have a deviation from the exponential behaviour. Freeman considered the Type II as cases where near the galaxy centre the extrapolated surface brightness of the disc is greater than the actual disc surface brightness. Essentially he described the down-bending disc breaks for the first time. Studying edge-on galaxies, and reaching lower surface brightness levels, van der Kruit (1979) discovered that the exponential decrease of surface brightness in the outer discs does not continue indefinitely. Instead, the surface brightness appears to truncate rather sharply at the very outer regions. These truncations were studied thoroughly in a series of papers examining the surface brightness profiles of edge-on galaxies (van der Kruit and Searle 1981a,b, 1982a,b). Work carried out with modern CCD-imaging in optical and infrared wavelengths further investigated the truncations in edge-on galaxies (Barteldrees and Dettmar 1994; Näslundand Jörsäter1997; Pohlen et al. 2000; de Grijs et al. 2001; Florido et al. 2001; Kregel et al. 2002). It became evident that the truncations are not always extremely sharp, but instead some are better described with two exponential sections, separated by a smooth transition. In addition, it was found that not all galaxies show such features in their disc, and seem to have exponential discs as far as the profiles can be traced. The studies of truncations also showed that the common measure used for the location of the truncation, the truncation radius divided by the inner disc scalelength (Rmax/hin), has a large scatter. However, it is correlated with the rotation velocity and thus the mass of the galaxy. Kregel and van der Kruit (2004) report that in their sample of 34 edge-on galaxies, the average value of this measure is Rmax/hin = 4.0±1.1. Similar value was found earlier by van der Kruit and Searle (1982a), but for example Pohlen et al. (2000) obtained a much lower value of 2.9 ± 0.7. The down-bends have also been observed in the vertically extended thick discs in edge-on galaxies (Comerón et al. 2012). However, edge-on view to galaxies is problematic because the de-projected surface brightness distribution is heavily model dependent, for

29 example, depending on how the dust in the galaxy mid-plane is taken into account (e.g. Pohlen et al. 2007). In addition, the down-bending features cannot be connected with morphological structures in edge-on galaxies. Pohlen et al. (2002) was the first to report that the down-bends in surface brightness profiles are also seen in the outer discs of three face-on galaxies (for example, galaxy in Fig. 3.1 has a down-bend at ∼ 120 arcsec). The down- bending profiles were further extensively studied by P06, E08, and G11, in high mass galaxies with large variety of morphologies (−3 . T . 8). These studies showed that the down-bending profiles especially in face-on galaxies are better described with two exponential sections, with a smooth transition. Furthermore, P06 showed that in face-on galaxies the average value of Rmax/hin ≈ 2, and that the individual Rmax/hin values are uncorrelated with the maximum rotational velocity of the galaxies. Down-bending disc profiles have been also observed in face-on late-type dwarf galaxies by Hunter and Elmegreen (2006), Hunter et al. (2011), and Herrmann et al. (2013), and in early-type dwarfs in the Virgo cluster by Janz et al. (2014), showing that they are not features present only in high mass disc galaxies. Are the down-bending features seen in the surface brightness profiles of edge-on and face-on galaxies the same? The radius of these features in kilo- parsecs among galaxies with similar luminosities seems to be comparable in edge-on and face-on galaxies according to some studies (e.g. Comerónet al. 2012). However, the average value of Rmax/hin appears to differ between edge-on and face-on samples. Mart´ın-Navarro et al. (2012) investigated this discrepancy, and showed that edge-on disc galaxies often posses two down- bending features: an inner break, followed by a sharper truncation further out in the galaxy (see Fig. 3.3). Furthermore, the inner break was found to be at similar radii as usually found in face-on galaxies, while the truncations resides at similar radii as normally observed in edge-on studies. Mart´ın- Navarro et al. found also that their measured truncation radius correlates with the maximum rotation velocity of the galaxy, similarly as found in previous studies of edge-on galaxies. The breaks appear to have significantly weaker correlation, as also found by P06 for moderately inclined galaxies. These differences point to a different nature of the breaks and truncations. Mart´ın-Navarro et al. (2012) argued that the truncations are probably drops in the real stellar mass density, associated with the maximum specific angular momentum of the galaxy. This was already proposed by van der Kruit (1987), who suggested that the truncations are imprints of maximum angular momentum of the proto-galaxy, and as such result from the formation processes of galaxies. On the other hand, breaks could be formed by evolutionary processes of the disc, independent of the initial galaxy formation. Recently, using extremely deep optical imaging of the galaxy UGC 00180 (inclination i ∼ 70◦), Trujillo and Fliri (2016) revealed that both break and truncation are visible in the surface brightness profile of this non-edge-on

30 NGC3501

40 20 0

[arcsec] -20 -40

20 ] -2

[AB mag arcsec Break 3.6 µ 26 Truncation 28 -150 -100 -50 0 50 100 150 [arcsec]

Figure 3.3: Edge-on galaxy NGC 3501 with the 3.6 µm radial surface brightness proﬁle, determined from a 20 pixel wide cut along the major axis of the galaxy (shown with white solid lines). In the surface brightness proﬁle and image the radii of the break and truncation are shown separately, with the locations of these features taken from the measurements of Mart´ın-Navarro et al. (2012).

−2 galaxy. The break in UGC 00180 is located at µr ≈ 22 AB mag arcsec , while the truncation is at much lower surface brightness level of µr ≈ 26 AB mag arcsec−2. The surface brightness at the break in UGC 00180 is where the breaks are commonly found in many studies of the down-bending proﬁles in face-on galaxies (e.g. P06, E08, G11). The truncations at the outer edges of galaxies are more easily observed in edge-on projection due to the line of sight integration (see also Mart´ın-Navarro et al. 2014). In summary, the down-bending features in face-on galaxies are more commonly known as breaks, and the sharper features further out in the discs usually seen in edge-on galaxies are truncations (illustrated in Fig. 3.3). Such a

31 division complicates the comparison of the properties between edge-on and face-on galaxies, because it is uncertain if the features addressed in edge-on studies are breaks or truncations. To overcome this Mart´ın-Navarro et al. (2012) suggested that the value of Rmax/hin could be used to separate these features. The breaks have lower values as they are in smaller radius, and compared to the truncations the disc scalelength inside the break is larger.

3.3.2 Down-bending breaks in face-on galaxies In moderately inclined high mass galaxies the down-bending, or Type II, breaks have been found to be present even in 50 ± 4% of the galaxies, being more frequent in the late-type disc galaxies (4 . T . 8, P06, E08, G11). Starting from P06 study, the down-bending profiles have also been separated into several categories, by their association with different structure components of the galaxies. The down-bending profiles have also been observed out to redshift z ∼ 1 in moderately inclined galaxies (Pérez2004; Trujillo and Pohlen 2005; Azzollini et al. 2008b), suggesting that they are persistent features. In addition, the location of the down-bend break radius evolves with redshift. Azzollini et al. (2008b) showed that the surface brightness at the break decreases even by 3.3 mag arcsec−2 in the rest-frame B-band, and the break moves radially out by a factor of 1.3, when comparing galaxies with similar masses across the redshift range of z ∼ 0 − 1. Azzollini et al. (2008b) conclude that their results are compatible with inside-out growth of galaxies, which causes considerable evolution of the inner disc. The observation of the Type II profiles has also been extended to dwarf galaxy mass regime. In fact, Herrmann et al. (2013) have found that even ∼ 60% of late-type dwarfs have such a disc profile. The face-on Type II breaks have also been extensively studied in stellar age and colour profiles (e.g. g’-r’, Bakos et al. 2008). The Type II mean colour and age profiles indicate an inside-out blueing or younger stellar ages, with the bluest colours and youngest stars found at the break radius. Beyond the break radius the colour reddens as the stellar populations become older. This characteristic U-shape profile has been found by several authors, using varying samples and techniques (e.g. Bakos et al. 2008; Azzollini et al. 2008a; Yoachim et al. 2010, 2012; Roediger et al. 2012; Zheng et al. 2015; Marino et al. 2016). Exceptions are also found, and some Type II discs have a flat colour and age profiles, which are usually associated with single exponential discs (Yoachim et al. 2012; Roediger et al. 2012; Ruiz-Lara et al. 2016). Moreover, it is not clear why the colour and age profiles vary from galaxy to galaxy. The samples of the colour and age profile studies have focused on large part on intermediate or late-type spiral galaxies, and no indication of morphological connection with the shape of the colour/age profiles has been given.

32 The diﬀerent categories of Type II breaks are introduced below, with their proposed formation mechanisms discussed in the literature. In this thesis (see papers I and IV) this sub-classiﬁcation of break types is not used and down- bending breaks are generally referred as Type II. Exception is made with Type II.i, which shows characteristics of both single exponential and Type II. However, we do connect the Type II breaks with structure components of the galaxies in papers I and IV, and in section 6.2.

Type II.i The Type II.i was introduced ﬁrst in P06 as down-bending break which is seen inside, or at the bar radius (see Fig. 3.4). Such galaxies show a single exponential surface brightness proﬁle outside the break. The extrapolated outer disc surface brightness is higher than the actual disc surface brightness near the centre, similarly to the original Type II characterisation of Freeman (1970). The Type II.i breaks are most likely formed by the bar formation and evolution, causing redistribution of material near the galaxy centre. Similar features are seen also in some N-body simulations (e.g. Athanassoula and Misiriotis 2002; Valenzuela and Klypin 2003). This type of break has received relatively little attention in the literature, and more thorough investigation could reveal information about the bar properties.

Type II-OLR Moving to larger radii, P06 and E08 found that some galaxies have a down- bending break at 2-2.5 times the bar radius. This is the typical location of the bar’s Outer Lindblad Resonance, and hence these breaks were named Type II-OLR (examples shown in Fig. 3.5). Consequently, this is the location where outer rings are usually located in galaxies (see section 2.4.2), and in many cases the break was directly associated with this structure (P06, E08, also papers I and IV). The outer rings are more common in the early-type galaxies (Buta and Combes 1996; Laurikainen et al. 2013; Comer´onet al. 2014; Herrera-Endoqui et al. 2015), and more II-OLR breaks were also found in the early-type disc galaxies (i.e. T < 2, P06, E08, G11, and paper I). The radial redistribution of gas and stars in the disc induced by the bar (see section 2.4.2) could further change the properties of the break as the galaxy evolves. In paper I, and section 6.2, we provide conclusive evidence that the outer ring structures and some Type II breaks are intimately connected. Fundamentally the outer rings are produced by star formation activity, born out of the gas accumulated at the bar resonances (see section 2.4.2). Even in the early-type S0 galaxies, which are normally thought to be devoid of star formation, outer-ring morphologies are observed in the far-ultraviolet light tracing recent star formation (e.g. Salim et al. 2012).

33 NGC5383 - log10 ( M* / MO • ) = 10.80 NGC5383 100 16

50 ] -2 20

0 22

[AB mag arcsec 24 3.6 -50 µ

Bar radius -100 28 -100 -50 0 50 100 0 20 40 60 80 100 Semi-major axis [arcsec]

NGC1187 - log10 ( M* / MO • ) = 10.43 NGC1187 16

150 18 100 ] -2 20 50

0 22

-50 [AB mag arcsec 24 3.6 µ -100 26 -150 Bar radius 28 -150 -100 -50 0 50 100 150 0 50 100 150 Semi-major axis [arcsec]

NGC2543 - log10 ( M* / MO • ) = 10.42 NGC2543 100 16

18 50 ] -2 20

0 22

[AB mag arcsec 24 3.6 µ -50 26

Bar radius -100 28 -100 -50 0 50 100 0 20 40 60 80 100 Semi-major axis [arcsec]

Figure 3.4: Same as Fig. 3.2, but Type II.i examples are shown.

34 NGC0936 - log10 ( M* / MO • ) = 10.93 NGC0936 16 200 18 150 ]

100 -2 20 50

0 22

-50 [AB mag arcsec 24 3.6

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-150 26 -200 Bar radius Break 28 -200 -150 -100 -50 0 50 100 150 200 0 50 100 150 200 Semi-major axis [arcsec]

NGC4593 - log10 ( M* / MO • ) = 10.93 NGC4593 16 150

18 100 ] -2 50 20

0 22

-50 [AB mag arcsec 24 3.6 µ

-100 26

-150 Bar radius Break 28 -150 -100 -50 0 50 100 150 0 50 100 150 Semi-major axis [arcsec]

NGC1672 - log10 ( M* / MO • ) = 10.66 NGC1672 16 200

150 18

100 ] -2 20 50

0 22

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-100 µ

-150 26 -200 Bar radius Break 28 -200 -150 -100 -50 0 50 100 150 200 0 50 100 150 200 Semi-major axis [arcsec]

Figure 3.5: Same as Fig. 3.2, but Type II-OLR examples are shown. The break radius is marked with a solid red line in the image and the surface brightness proﬁle.

35 Lens structures are characterised by nearly constant surface brightness, with a rather sharp outer edge. Based on their description it is natural that they would manifest as down-bending breaks in the surface brightness profiles of galaxies. Indeed, in paper I we found that some of the down-bending breaks in early-type galaxies are connected with outer lenses (shown in more detail in section 6.2). These structures are mainly found in the early-type S0 galaxies (e.g. Laurikainen et al. 2013; Herrera-Endoqui et al. 2015), and the sizes of outer lenses are similar to outer rings. One of the hypothesis of outer lens formation is indeed through dynamical heating of outer-rings (see Laurikainen et al. 2013 and references therein). Intermediate class of outer ringlenses also exists, which shows characteristics of both structures. The outer ringlenses are similarly connected with some of the Type II breaks. Erwin et al. (2012) has noted that S0 galaxies in the Virgo cluster seem to have a distinct lack of Type II down-bending breaks, compared to S0 galaxies in the field environment (see also Roediger et al. 2012; Maltby et al. 2015). The fraction of Type II profiles does indeed reduce in the T . −2 S0 galaxies (e.g. paper I), but S0 galaxies in Virgo are not completely devoid of Type II profiles (see paper I). Erwin et al. (2012) argue that the reduced fraction of Type II breaks in Virgo could be a manifestation of gas removal from the galaxies, which would reduce the occurrence of outer rings in the cluster galaxies. Indeed, the fraction of outer rings decreases towards the T = −3 S0 galaxies, which do not have outer rings but commonly show outer lens structures as found by Laurikainen et al. (2013), but the galaxies in Laurikainen et al. appear outside any clusters (only a few are part of Virgo cluster). Lastly, it is important to note that the presence of an outer ring or lens structure in a galaxy is not a definite evidence for a Type II break to be present in the galaxy. Many galaxies with outer rings show a single exponential surface brightness profile. Thus, the presence of a break feature could possibly be used to estimate the prominence of the resonance structure it is associated with.

Type II-CT In unbarred galaxies, further out in the disc than the outer ring, or when no resonance structures are seen, P06 classiﬁed the down-bends as Type II-CT, or Type II Classical Truncation (examples shown in Fig. 3.6). The naming refers to the truncations seen in edge-on galaxies, as ﬁrst noticed by van der Kruit (1979), which truncations were thought to resembled the Type II-CT breaks in face-on galaxies. P06 showed that the fraction of these breaks is largest in the late-type disc galaxies (i.e. T > 5, see also paper I), where the fraction of outer rings and lenses is also lower. The location of the II-CT breaks in spiral regions suggests a star formation

36 NGC5964 - log10 ( M* / MO • ) = 10.26 NGC5964 16 150

18 100 ] -2 50 20

0 22

-50 [AB mag arcsec 24 3.6 µ -100 26

-150 Bar radius Break 28 -150 -100 -50 0 50 100 150 0 50 100 150 Semi-major axis [arcsec]

NGC7171 - log10 ( M* / MO • ) = 10.58 NGC7171 100 16

50 ] -2 20

0 22

[AB mag arcsec 24 3.6 -50 µ

Bar radius Break -100 28 -100 -50 0 50 100 0 20 40 60 80 100 Semi-major axis [arcsec]

ESO362-009 - log10 ( M* / MO • ) = 8.90 ESO362-009 16 150

18 100 ] -2 50 20

0 22

-50 [AB mag arcsec 24 3.6 µ

-100 26

-150 Break 28 -150 -100 -50 0 50 100 150 0 50 100 150 Semi-major axis [arcsec]

Figure 3.6: Same as Fig. 3.2, but Type II-CT examples are shown. The break radius is marked with a solid red line in the image and the surface brightness proﬁle.

37 related origin. Indeed, models have shown that a star formation threshold in the disc could cause the formation of a break (Kennicutt 1989; Schaye 2004; Elmegreen and Hunter 2006; Roˇskar et al. 2008b; Sánchez-Blázquez et al. 2009; Mart´ınez-Serrano et al. 2009). Ultimately the manifestation of a star formation threshold would be due to a drop in the gas density, which causes the gas to be stable against collapse into dense clouds. If the break is formed by such a star formation threshold, the outer disc beyond the break would be populated by relatively few stars, assuming that the galaxies grow in inside-out fashion. However, it is suggested that the disc beyond the break is populated by stars which have slowly radially migrated to a large radius (see section 2.4.2). Such a combination of star formation threshold origin of the break, complemented by radial migration of old stellar population, would explain the observed U-shaped colour profiles described previously. The youngest stars and bluest colours would be seen at the break radius, corresponding to the position of the star formation threshold in the disc. The outer disc reddening and increasing stellar population age would be caused by the radially migrated stars (e.g. Roˇskar et al. 2008b). As mentioned previously, studies of z ∼ 1 galaxies have revealed that the position of the break is different compared to galaxies in the nearby universe. This evolution in the break position could be explained by the changing radius of the star formation threshold as the galaxy grows inside-out (Azzollini et al. 2008b). Some of the breaks further out in the disc than 2-2.5 times the bar radius might also have an alternative formation mechanism. Simulations have shown that down-bending break formation is coupled with the bar formation, through bar-spiral resonant coupling in the disc (Debattista et al. 2006). Ob- servational support for this process has been found by Muñoz-Mateoset al. (2013). They found a group of Type II breaks located at ∼ 3.5 times the bar radius, at which radius one of the spiral pattern resonances was found to overlap with the bar corotation radius. Foyle et al. (2008) extended simulations of Debattista et al. (2006), and argue that down-bending Type II breaks form only when md/λ > 1, where md is the disc-to-total mass ratio and λ is non-dimensional halo spin parameter (see also Herpich et al. 2015). When this ratio decreases the disc becomes more stable, which suppresses angular momentum redistribution and the break formation in the disc. The radial location of the Type II break would also move outwards for larger md/λ ratios, if the criteria for the star formation threshold remains the same. Foyle et al. (2008) noted based on their simulations that the value of concentration index C28 = 5 log(r80/r20), where r80 and r20 are the radii containing 80% and 20% of the baryonic mass, could be used as a criteria for break formation. They found that after 10 Gyr evolution all the simulated galaxies with breaks had a value of C28 & 3.5. Observational support for this has not been found, and results of Muñoz-Mateos et al. (2013) show that galaxies with or without a break show no connection with the value of C28 index.

38 Type II-AB The fourth and final class of down-bending breaks in face-on galaxies is the Type II-AB, also proposed by P06, which comprises of cases that show breaks caused by the asymmetry (NGC 5964 in Fig. 3.6 could be classified as an intermediate case between II-CT and II-AB). The Type II-AB classification is used when the disc is clearly lopsided, and shows for example extended spiral arms in one side of the disc. The break is then assumed to form from the uneven light distribution in the elliptical isophotes, over which the radial surface brightness profile is constructed. P06 and E08 noted that Type II-AB seem to be present only in the late-type spiral galaxies.

3.4 Up-bending disc breaks

In addition to the down-bending surface brightness profiles, some galaxies show an opposite behaviour: in outer regions the slope of the surface brightness is shallower than in the inner parts, as first found by Erwin et al. (2005) (examples shown in Fig. 3.7). Therefore, these features were named “an- titruncations”, nowadays also known as Type III up-bending breaks. Com- pared to Type II, the Type III up-bends are located at slightly larger radii (e.g. Type II at 0.79 R25 and Type III at 1.02 R25, G11). These features were reported by G11 to be present in 38 ± 4% of their sample galaxies, and have also been observed in z ∼ 1 galaxies (e.g. Trujillo and Pohlen 2005; Azzollini et al. 2008b). Erwin et al. (2005), and soon after P06, E08, described that the up- bending profiles can be divided into two classes: cases which show a gradual transition from the inner slope to the more shallow outer slope (“III-s”), and cases with a rather sharp transition (“III-d”). Erwin et al. (2005) also noted that the III-s cases have rounder elliptical isophotes in the outer regions, and might thus be related to the spherical stellar halo component of the galaxies. On the other hand, the Type III-d does not exhibit change in the outer disc ellipticity, and would be true disc phenomena. Some Type III-d profiles were indeed noted to show spiral structure also beyond the break. In face-on galaxies the division using the elliptical isophotes of the outer disc is challenging, as the discs themselves are thought to posses nearly circular geometry when viewed face-on. Furthermore, the smoothness of the break is subjective, and no good measures exist. These difficulties were already discussed by P06, in which study they resorted to classify up-bending disc profiles generally as Type III. Exceptions were made in cases where the outer disc showed distinct spiral structure, in which case the profile was classified as III-d. In this thesis (see papers I and IV) we do not use this sub-classification of the up-bending profiles, and simply classify them as Type III. Formation of the up-bending profiles has been mainly attributed to envi-

39 NGC3485 - log10 ( M* / MO • ) = 9.94 NGC3485 16 100

50 ] -2 20

0 22

[AB mag arcsec 24 3.6

-50 µ

-100 Bar radius Break 28 -100 -50 0 50 100 0 20 40 60 80 100 Semi-major axis [arcsec]

NGC7742 - log10 ( M* / MO • ) = 10.34 NGC7742 16 100

50 ] -2 20

0 22

[AB mag arcsec 24 3.6

-50 µ

-100 Break 28 -100 -50 0 50 100 0 20 40 60 80 100 Semi-major axis [arcsec]

NGC3684 - log10 ( M* / MO • ) = 10.18 NGC3684 16 100

50 ] -2 20

0 22

[AB mag arcsec 24 3.6

-50 µ

-100 Bar radius Break 28 -100 -50 0 50 100 0 20 40 60 80 100 Semi-major axis [arcsec]

Figure 3.7: Same as Fig. 3.2, but Type III examples are shown. The break radius is marked with a solid red line in the image and the surface brightness proﬁle.

40 ronmental processes. Before the Type III profiles were thoroughly examined, the up-bending light profiles were noted in a study of M51-type interacting galaxies by Laurikainen and Salo (2001). Indeed, as discussed in section 2.4.1, minor-merging events can spread out the disc in radial direction, and thus cause an up-bending in the surface brightness distribution. The environmental processes have since been demonstrated with simulations to produce up-bending profiles, in both minor (Peñarrubia et al. 2006; Younger et al. 2007), and major merging events (Borlaff et al. 2014). In addition, smooth gas accretion could disturb the outer stellar disc, and cause the formation of a Type III profile (section 2.4.1, Minchev et al. 2012). Observational studies have also attempted to find evidence of environmental origin of Type III profiles. Some galaxies with Type III profiles do indeed show strongly interacting morphology (e.g. the M51-type galaxies in Laurikainen and Salo 2001), but a majority of them are not identified as interacting. In their study, P06 counted the neighbouring galaxies, and found that on average galaxies with a Type III profile have more neighbours. However, their result was not conclusive and was hindered by rudimentary criteria for the environment, as well as by the lack of reliable environmental catalogue. Furthermore, Maltby et al. (2012a) compared the properties of galaxies in the field and cluster environments, but found no evidence of environmental effects regardless of the profile type. Head et al. (2015) found, using two dimensional decompositions of Coma cluster S0 galaxies, that galaxies with Type III profiles show an increase in the bulge size and luminosity. Their results are qualitatively compatible with a merger scenario where bulge growth is accompanied with a Type III break formation. In paper I we studied the environments of the sample galaxies by using the 2 Micron All Sky Survey (2MASS, Jarrett et al. 2000) and 2MASS Redshift Survey (Huchra et al. 2012). We first calculate the surface number density of galaxies: 3 Σ3 = log10 2 , (3.1) πR3 where R3 is the projected distance in Mpc to the third nearest neighbour galaxy, with recession velocity of ±1000 km s−1 around the primary galaxy. Logarithm is used in this parameter because of the large variation of this parameter among the galaxies. Values of the so called Dahari-parameter (Da- hari 1984) are also calculated for the neighbourhoods of the sample galaxies. The Dahari-parameter estimates the tidal force produced by the companion galaxy divided by the internal binding force of the primary galaxy, defined as: −1 γ 3 Ftidal Mc Dp Mp Dc Dp Qi = = 3 2 = , (3.2) Fbinding S Dp Dp S where Dp and Dc are the diameters and Mp and Mc are the masses of the primary and companion galaxies, and S is their projected separation. The

41 value of γ is the proportionality between the galaxy mass and its radius, which is not well known. In paper I we adopted γ = 1.5 (Rubin et al. 1982; Dahari 1984; Verley et al. 2007). The values for the galaxy diameters were collected from 2MASS, using the semi-major axis length of the ellipse at the −2 isophote of Ks = 20 mag arcsec (r k20fe) multiplied by two. Following Verley et al. (2007), Q is the logarithm of the sum of all Qi:

n ! X Q = log10 Qi , (3.3) i=1 where n is the number of galaxies found within projected separation of 1 Mpc, and with recession velocity of ±1000 km s−1, around the primary galaxy. We found a correlation between the Dahari-parameter values, and the inner and outer disc scalelengths of Type III profiles: the disc scalelengths are larger for larger Dahari-parameter values. Similar correlation was also found for the Type I single exponential discs of early-type galaxies, which on average also reside in more dense environments. However, no clear correlation was found when using the surface number density of galaxies. Altogether, the observations indicate that at least some fraction of the up-bending profiles are caused by environmental effects. Alternative formation processes have also been proposed. In very low −2 surface brightness levels of µr ∼ 28 mag arcsec the stellar halo could systematically produce an up-bending profile, as shown by Bakos and Trujillo (2012), Mart´ın-Navarro et al. (2014), and Trujillo and Fliri (2016). The up- bending breaks have also been observed in the vertically extended thick discs. In fact, Comerónet al. (2012) proposed that even up to ∼ 50% of Type III up-bends in face-on galaxies could be due to superposition of a thin and thick disc, when the scalelength of the thick disc is larger than that of the thin disc. Moreover, bars seem not to play a significant role in the formation and evolution of Type III profiles. This is indicated by Eliche-Moral et al. (2015), who compared various scaling relations of Type III profiles, and found no significant differences between galaxy types, or barred and unbarred galaxies. Extended UV emission, indicating star formation beyond the optical disc, has been found in many galaxies, possibly ignited by gas inflow to the galaxy (see section 2.4.1). In such cases the increased star formation at the outskirts of galaxy could give rise to some of the observed Type III profiles. However, no statistical studies exist linking the up-bending Type III profiles with extended UV emission. Even without gas accretion or other external processes, star formation could in principle create some of the up-bending profiles. Elmegreen and Hunter (2006) showed, with semi-analytical models, that suitable star formation conditions could indeed result in the formation of up-bending profile. Lastly, Maltby et al. (2012b) has suggested that even 15 % of the up-bending profiles could be explained by an excess of bulge

42 light at large radii. However, this is based on very uncertain extrapolation of bulge light much beyond the radii where the bulge is actually ﬁtted.

3.5 Open questions addressed in this thesis

Multitude of processes are evolving the discs of galaxies, and as revealed by numerous observational and simulation studies, these processes can manifest as deviations from a single exponential behaviour of the disc surface brightness. Combinations of the two main break types (e.g. Type II + III) have also been observed in galaxies, showing that different processes can simultaneously modify the disc light distribution. Perhaps the most conclusive formation processes have been presented for the down-bending Type II breaks. However, the formation processes of Type III profiles and single exponential discs remain poorly understood, with many alternative formation mechanisms proposed. A series of papers by P06, E08, and G11 were the first to report overall fractions of the different profile types, over a large range of morphological types, in high mass galaxies. More recently, Herrmann et al. (2013) has expanded the disc profile studies also to late-type and low mass dwarf galaxies, showing that Type II down-bending breaks dominate in these galaxies. However, these studies have left region of intermediate mass disc galaxies unstudied. We have aimed to cover this moderate mass galaxy regime, and examine if stellar mass affects the profile types. In section 6.1, and in paper IV, the effect of stellar mass on the profile types is discussed. The classification of the different disc profile types relies on visual in- spection, based on fits to the disc profiles, discussed in chapter 5. A critical question is, how large deviations from the single exponential behaviour should be considered as Type II or III discs? This has not been addressed in the literature, and naturally some differences are to be expected between different studies. In section 6.1 we compare our results to some previous studies, in order to see whether systematic differences could appear between the studies. The different Type II subclasses are intimately connected with structure components of the galaxy discs. Therefore, to further constrain the formation mechanisms of the Type II breaks it is essential to examine how the galaxy structures are connected with the break radius. This has been done previously by P06, E08, and G11, and here we have expanded these studies to also consider the galaxy stellar mass. In section 6.2 (and papers I and IV) we have systemically connected the Type II break location with measurements of the structure components from literature, and show that the Type II breaks can be classified to two major categories as previously proposed. The disc breaks have been mainly studied using optical data, in which the recent star formation and dust features are more pronounced. Infrared

43 wavelengths are dominated by light from the low mass main sequence stars, which comprise most of the stellar mass of galaxies. Only recently infrared data has been used to study the disc profiles in a similar manner as in the optical (e.g. Herrmann et al. 2013; Muñoz-Mateoset al. 2013; Kim et al. 2014, and papers I and IV). Comparing the properties of breaks between optical and infrared can be used to roughly estimate how the youngest and oldest stellar populations change the break properties. For example, the formation of the major Type II classes, II-OLR and II-CT, is ultimately assumed to be due to star formation in the inner disc. However, the outer discs might be affected in completely different manner, due to the break formation mechanism: bars in Type II-OLR, and star formation threshold in II-CT. In paper IV we have compared the disc and break parameters using a large sample of galaxies with data in six different wavelengths. We present the results in section 6.4.

44 Chapter 4

Data and sample selection

The data used in this thesis comprises of mid-infrared images at 3.6 µm from the Spitzer Survey of Stellar Structure in Galaxies (S4G), and 2.2 µm images from the Near Infrared S0-Sa galaxy Survey (NIRS0S). In this chapter these surveys, and additional optical data from Sloan Digital Sky Survey, are described. The sample selection criteria of papers I and IV is also presented.

4.1 Data

Spitzer Survey of Stellar Structure in Galaxies (S4G) The Spitzer Survey of Stellar Structure in Galaxies (S4G, Sheth et al. 2010), is a science legacy program of the Spitzer space telescope (Werner et al. 2004). The survey utilizes the post-cryogenic phase of the telescope in the 3.6 and 4.5 µm wavelengths of the Infrared Array Camera (IRAC) instrument (Fazio et al. 2004). The sample selection of S4G comprises all galaxies which fulﬁl the following criteria in HyperLeda (Makarov et al. 2014):

radial velocity Vradio < 3000 km/s,

total corrected B-band magnitude mBcorr < 15.5,

B-band isophotal angular diameter D25 > 1 arcmin, galactic latitude |b| > 30◦. The total observed sample comprises of 2352 galaxies, over a wide range of morphologies and stellar masses (see Fig. 4.1). Use of the HI radial velocity (i.e. radio measured recession velocities) omits gas poor early-type galaxies. The scarcity of early-type galaxies is also visible in Fig. 4.1. All the galaxies in the sample were observed in both wavelengths (3.6 and 4.5 µm), and the images reach azimuthally averaged stellar mass surface densities of −2 ∼ 1 M pc , with a pixel resolution of 0.75 arcsec/pixel. The Spitzer archival

45 data was also utilized in the survey, and for example data obtained for the SINGS survey (Kennicutt et al. 2003) was used. The wavelengths used in the survey, 3.6 and 4.5 µm, are not sensitive to internal extinction of galaxies (Draine and Lee 1984) and trace the old stellar population. However, minor contaminations from hot dust, polycyclic aromatic hydrocarbons (PAHs), and bright intermediate age stars with low mass-to-luminosity ratio (M/L) are also present (Meidt et al. 2012). How- ever, the M/L-ratio is nearly constant in the 3.6 and 4.5 µm wavelengths (Peletier et al. 2012), and the images are good tracers of the stellar mass distribution in galaxies. Data reduction and analysis in the S4G consists of four pipelines. Pipeline 1 creates the science ready mosaics from the individual frames. Pipeline 2 generates mask images of the foreground stars and image artefacts, which are essential for the analysis of the 3.6 and 4.5 µm images of S4G. Initial masks are created using SExtractor software (Bertin and Arnouts 1996), which are then edited by hand to increase the accuracy especially in the area of the galaxies (paper II). The Pipeline 3 first measures the background level and determines the galaxy centre using IRAF task imcentroid, followed by elliptical isophote fits of the galaxy. The isophote fits are then used to construct the radial profiles of ellipticity, position angle, and surface brightness. Fi- nally, Pipeline 3 calculates the position angles and ellipticities of the outer discs, asymptotic magnitudes, galaxy sizes, stellar masses, and concentration indices of the galaxies (paper II). The final Pipeline 4 decomposes the light distribution of the galaxies into different structural components, such as bulges, bars, discs, and if needed a central point-source (paper III). The S4G images and pipeline data products have been released in the NASA/IPAC Infrared Science Archive1. Extension of the S4G survey, which will add 695 elliptical and S0 galaxies to the survey, is currently underway (Sheth et al. 2013).

Near Infrared S0-Sa galaxy Survey (NIRS0S) The Near Infrared S0-Sa galaxy Survey (NIRS0S, Laurikainen et al. 2011) is a survey of early-type disc galaxies in the 2.2 µm Ks-band, conducted on 3-4 meter class telescopes. The sample selection for the survey, described in Laurikainen et al. (2011), is based on the Third Reference Catalogue of Bright Galaxies (RC3, de Vaucouleurs et al. 1991) and is done with the following criteria:

Hubble type −3 ≤ T ≤ 1,

total B-band magnitude mB ≤ 12.5,

1http://irsa.ipac.caltech.edu/data/SPITZER/S4G/

46 -5 0 5 10 300 4 200 S G

N 100 NIRS0S

0 12

11 11

) 10 10 • O /M * 9 9 (M 10 8 8 log 7 7

6 6 -5 0 5 10 0 100 200 300 Hubble type N

Figure 4.1: The stellar mass as a function of morphological type for the galaxies in the S4G and NIRS0S surveys. For S4G galaxies the mid-IR morphological types are provided by Buta et al. (2015), and the stellar masses by paper II. For NIRS0S galaxies the mid-IR morphological types are taken from Laurikainen et al. (2011), and the stellar mass calculation of NIRS0S galaxies is explained in section 4.1.

inclination i < 65◦.

Slightly fainter additional galaxies are also included in the NIRS0S sample, and in total the full sample contains 215 galaxies (13 ellipticals, 139 S0s, 30 S0/a, 33 Sa, and 1 later type galaxy). NIRS0S sample partly overlaps with the S4G survey, having 93 galaxies in common. The S4G data is on average two magnitudes deeper than the NIRS0S data (e.g. Fig. 4 in Laurikainen et al. 2011). In addition, the NIRS0S data is calibrated using the 2 Micron All Sky Survey (2MASS, Jarrett et al. 2000) Vega magnitude system, rather than the AB system used in S4G. In paper I we compared the aperture magnitudes between the NIRS0S Ks and the S4G 3.6 µm images, of the 93 galaxies which appear in both surveys. We found that the median diﬀerence in the aperture magnitudes is 2.566, and the relation is highly linear. This value can be used to convert the NIRS0S Vega magnitudes to the 3.6 µm AB system of the S4G. Furthermore, the M/L-ratio reported for 3.6 µm data by Eskew et al. (2012) can be used to calculate the stellar masses of the NIRS0S galaxies. In Fig. 4.1 the NIRS0S galaxies stellar mass is shown as a function of the mid-IR morphological type. NIRS0S provides an important addition to the S4G sample among the

47 early-type galaxies.

Sloan Digital Sky Survey (SDSS) The Sloan Digital Survey (SDSS) is a large survey that covers approximately one third of the full sky. It uses a dedicated 2.5 meter telescope, and provides imaging in five different filters: u, g, r, i, and z (Smith et al. 2002), with a pixel scale of 0.396 arcsec/pixel. The background in the SDSS images is uniform, and in the best cases enables the study of the azimuthally averaged −2 surface brightness profiles down to µr ∼ 27 mag arcsec (e.g. P06, paper IV). This surface brightness level is comparable to the depth of the 3.6 µm data of the S4G (paper IV). Knapen et al. (2014) have prepared SDSS imaging data in the five different filters from Data Releases 7 and 8 (DR7 , DR8) for 1768 galaxies that appear in the S4G survey.

4.2 Sample selection

Paper I In paper I we studied the properties of disc breaks in high mass galaxies, and the eﬀects of environment on the discs and breaks. The sample was constructed from the S4G and NIRS0S surveys for this purpose with the following criteria:

mid-IR Hubble type −3 ≤ T ≤ 7 (Buta et al. 2015; Laurikainen et al. 2011),

−2 2MASS Ks-band 20 mag arcsec isophotal aperture magnitude m ≤ 9.5 (Jarrett et al. 2000),

minor/major axis ratio of the outer disc b/a > 0.5.

Initially in this study the late-type disc galaxies (T > 7) were excluded because it was uncertain how reliably the radial surface brightness proﬁles could be studied. In addition, the used Ks-band magnitude limit (i.e. mass limit) restricts the amount of galaxies with T > 7, and the statistics in these galaxies would be very poor (see Fig. 4.2). Analysis done later in paper IV proved that also in the late-type disc galaxies the surface brightness proﬁles could be reliably studied. However, the magnitude limit in paper I was chosen because of the ∼ 97% completeness limit of the 2MASS Redshift Survey (Huchra et al. 2012), which is 11.75 mag in the Ks-band. By selecting the magnitude limit of m ≤ 9.5 the environment of the sample galaxies could be traced including companion galaxies which are two magnitudes fainter. The

48 inclination criteria restricts the sample to galaxies with inclination of 60◦ or less. The selection criteria returns 493 galaxies (336 from S4G, 103 from NIR- SOS). After inspecting the images 111 galaxies were excluded, for which the analysis of the surface brightness profiles would be unreliable. In these 111 galaxy images a bright foreground star was located near or inside the galaxy disc, image artefacts were found, or large gradients were present in the images. The final sample used in the study comprises of 328 galaxies, 248 from 4 S G using the 3.6 µm data, and 80 from NIRS0S using the Ks-band data. The sample of paper I has comparable Hubble type and MB ranges with the total sample studied in a series of papers using optical data by P06, E08, and G11 (see Fig. 4.3). However, the number of galaxies is higher, and for the first time the properties of all the disc profile types were studied using infrared data in a wide range of morphological types.

Paper IV In paper IV the study of paper I was extended to lower galaxy masses, and later Hubble types. The sample was again selected from S4G and NIRS0S but now using the following criteria: mid-IR Hubble type −2 ≤ T ≤ 9 (Buta et al. 2015; Laurikainen et al. 2011), and minor/major axis ratio of the disc b/a > 0.5. The bulge dominated very early-type disc galaxies (T = −3) were now excluded. This is because it is difficult to reliably separate the bulge and disc light using the surface brightness analysis methods in these galaxies with large bulges (see section 5.2.1). The latest disc galaxy types (T > 9) were also excluded because of their irregular morphology, which complicates the acquisition and analysis of the surface brightness profile. In total 952 galaxies fulfil the above criteria. However, after inspecting the data 193 galaxies were dismissed, due to same reasons described above in the sample selection of paper I. The final sample consists of 759 galaxies, of 4 which 43 have Ks-band data from NIRS0S, and 716 have 3.6 µm S G data. Of these galaxies 274 were included in the paper I sample. The Fig. 4.2 demonstrated that the sample of paper IV provides far better statistics for galaxies with Hubble types T & 2 compared to paper I. Additional optical data is used in paper IV, and it was added using the compilation of Knapen et al. (2014). In total 450 galaxies from the sample have data in the five different SDSS wavelengths. Knapen et al. (2014) also provide additional g-band data acquired with the Liverpool telescope, which was used for 30 galaxies. The distribution of the sub-sample with optical data is also shown in the histograms of Fig. 4.2.

49 -4 -2 0 2 4 6 8 10

120 Paper IV

80 Paper IV optical data N 40 Paper I 0 12

11 11 ) • O

/M 10 10 * (M

10 9 9 log 8 8

7 7 -4 -2 0 2 4 6 8 0 40 80 120 Hubble type N

Figure 4.2: The distribution of morphological types and stellar masses of galaxies in the paper I and IV samples.

120 120 Paper I Paper I Paper IV Paper IV 100 100 Pohlen & Trujillo 2006 Pohlen & Trujillo 2006 Erwin et al. 2008 Erwin et al. 2008 80 Gutierrez et al. 2011 80 Gutierrez et al. 2011 Herrmann et al. 2013 Herrmann et al. 2013 Munoz-Mateos et al. 2013 Munoz-Mateos et al. 2013

N 60 N 60

40 40

20 20

0 0 -5 0 5 10 -24 -22 -20 -18 -16 -14 -12 -10 Hubble type (HyperLeda) MB (HyperLeda)

Figure 4.3: Paper I and IV samples compared to previous studies which have examined the fractions of the diﬀerent disc proﬁle types.

50 The sample of paper IV bridges studies of the disc profile types between massive and low mass galaxies (see Fig. 4.3). Previous studies have separately focused on the bright high mass galaxies (P06, E08, G11, and paper I), and the low mass dwarf galaxies (Herrmann et al. 2013). The large distribution of the sample galaxies among morphological types and stellar masses enables the study of the different profile types as a function of galaxy stellar mass (see Fig. 4.2).

51 52 Chapter 5

Photometric decomposition methods

Decomposing galaxies to different structural components has become a widely used tool in the study of galaxy evolution. In the decompositions, analytical functions are used to fit the light distribution of galaxies and separate it to different physical structures, such as bulges, bars, and discs. The main attraction are the parameters returned from these fits, which describe the different components, and which can be used to perform detailed studies of the different structures. The decomposition methods can be divided into two classes: one and two dimensional methods. The former method uses one dimensional surface brightness profiles of galaxies, while the latter makes use of the full two dimensional information of a galaxy image.

5.1 General considerations

Preparation for the decompositions consists of basic data analysis, regardless of the decomposition method (e.g. paper III). First, point source masks of the images are created (papers II and III), followed by measuring galaxy centre and the background level in the images (papers II and III). The ellipticity and position angle of the outer disc are then measured, using elliptical isophote fits with free ellipticity and position angle (papers II and III). Ma- jor source of uncertainty in decompositions that affects especially the faint outskirts of galaxies comes from the background sky subtraction. This is vitally important in the determination of the disc profile type. In the images of the previous chapter (Figs. 3.2, 3.4, 3.5, 3.6, and 3.7) it can be noted that by over- or under-subtracting the sky by the standard deviation of the sky (σsky) the shape of the outer profile is changed. Poor background subtraction could thus also lead to misclassification of the disc profile type. In addition, the proper sky estimation is important especially for the disc parameters returned from the decompositions. However, given that the weighting scheme

53 used in the decompositions is correct and takes the background noise into account, the effect should be small. This is shown in paper III, where it was found that under- or overestimating the sky by the σsky has little effect on disc scalelength. The decomposition strategy is always chosen for the science goal. For example, in the pipeline decompositions of paper III, the goal was to retrieve reliable parameters describing the bulges and the discs. This prompted the use of more complicated models than which are generally used for such a large number of galaxies. Commonly in large galaxy samples only one or two components are used in the decompositions (i.e. single Sérsiccomponent or bulge and disc; Allen et al. 2006; Driver et al. 2006; Cameron et al. 2009; Lackner and Gunn 2012; Häußleret al. 2013). However, in the paper III maximum of four different components are used, which are bulge, disc, bar, and nucleus. Several different combinations can be produced from these four components, which are explained in paper III. Robustness is also a general rule in our decompositions: model components should be added only if simple models do not provide a satisfactory fit. Nevertheless, too simple models do not provide reliable parameters of the galaxies. This is especially important for barred galaxies, which are generally build from three equally important structure components: bulge, bar, and disc. Several studies have shown that the parameters of the structure components of galaxies obtained from decompositions are severely affected by the inclusion, or omission, of the bar component (e.g. Laurikainen et al. 2005, 2006, 2010; Gadotti 2008; Weinzirl et al. 2009, and paper III). The inclusion of a bar is particularly important for obtaining the parameters of the bulge reliably, such as the bulge effective radius, bulge Sérsicindex (see Eq. 5.1 below), and the bulge-to-total light ratio of the galaxy (B/T ) (paper III). Similarly, the inner lenses and barlenses commonly found especially in early-type disc galaxies can affect the parameters of the bulges, if not taken into account in the decompositions (Laurikainen et al. 2005, 2014). The weighting scheme used in the decomposition process can greatly affect the parameters derived from the fits. The weighting on decompositions is done in order to minimize the deviation between the real flux distribution of the galaxy and the model, by giving each data point a weight and calculating a “goodness-of-fit” value. In practise, a non-linear least squares algorithm is used to find the optimal model solution, when minimizing a statistical test value describing the difference between the data and the point spread function convolved model (e.g. Levenberg-Marquardt algorithm and χ2 test value, used both in two dimensional GALFIT software (Peng et al. 2002, 2010), and in one dimensional MPFIT routines (Markwardt 2009)). The weight in the fit process (wi) is generally tied to the statistical uncertainty of 2 the flux (wi ∼ 1/σi ), which consists of various components. To be specific, the Poisson noise contribution associated with the number of arriving photons

54 (object and sky background contribution) and instrumental noise. The exact sources of the noise components depend on the data reduction processes, and the observations. For example, in ground based optical observations the major noise component is the photon Poisson noise of the source, while ground based infrared observations are dominated by the thermal noise of the atmosphere, and possibly the instrument. In the paper III decompositions, contributions from the photon Poisson noise and the instrumental noise are both significant. The weighting scheme depends also whether one or two dimensional methods are used. Laurikainen and Salo (2000) examined the effect of different weighting schemes on the results of one dimensional bulge+disc decomposi- −2 −2 tions. Namely, the σi in point i, the surface brightness Σi in point i, total −1 flux within each annulus in the profile ΣAnnulus, and unweighed fits done in both flux and magnitude units. They found that the effect was mainly on the disc scalelength, when comparing the original fit and a fit made to a profile in which noise was added. Moreover, they reported that most robust results were obtained when the fit was made in magnitude units, and similar weight was given to individual data points. This is also the method how the one dimensional fits were performed in papers I and IV. In two dimensional methods the weights are usually given in the form of a σ-image, in which each pixel is a statistical estimate of the uncertainty of the flux in the same pixel of the data image. In paper III, the calculation of the σ-images for the 3.6 µm S4G data is explained in detail.

5.1.1 Radial profile functions The radial profile functions used in the decompositions have no strict physical justification. Instead, they have been empirically found to represent the light distribution of different structure components of galaxies, and when added together the total light distribution of a galaxy. The bulge is usually modelled with the Sérsicfunction (Sersic 1968): " !# r 1/n Σ(r) = Σe exp −κ − 1 . (5.1) re

Here, the index n controls the shape of the surface brightness distribution. High values of n (e.g. n > 4) result in a strong central peak and a wide shallow tail, and decreasing n results to a more gradually decreasing surface brightness. Low values of n (e.g. n < 1) result in a flatter and less concentrated surface brightness distribution. Half of the total flux is found inside the effective radius Re, and Σe is the surface brightness at that radius. The constant κ is tied with n by the requirement that half of the total flux is within the effective radius. The de Vaucouleurs r1/4-law (de Vaucouleurs

55 1948), often used for elliptical galaxies and sometimes also for bulges, is a special case of the S´ersicfunction with n = 4. As already introduced, Freeman (1970) found that the discs in galaxies can be described well with an exponential function: r Σ(r) = Σ exp − . (5.2) ◦ h

Here Σ◦ is the central surface brightness of the disc, h is the exponential disc scalelength. The exponential function is also a special case of the Sérsic function with n = 1. Erwin et al. (2008a) presented the broken exponential function, which can be used to fit discs with Type II or III surface brightness profiles: −r 1 1 − 1 h α(r−RBR) α h ho Σ(r) = S Σ◦ e i 1 + e i . (5.3)

Now Σ◦ is the central surface brightness of the disc section inside the break radius, defined by RBR. The disc scalelengths are hi and ho, where the designations i and o in the subscript refer to the inner and outer disc sections, separated by the break. The parameter α describes the smoothness of the break, with values 0 < α < 0.5 arcsec−1 resulting in a smooth transition, −1 and values of α & 0.5 arcsec in a quite abrupt transition. The value of α is fixed to α = 0.5 arcsec−1 in papers I and IV in order to reduce the number of free parameters in the fit. The value α = 0.5 arcsec−1 has been found to be a common value of the break smoothness (e.g. Erwin et al. 2008a, see also Comerónet al. 2012). We have run tests also with free value for α, but the parameters of the discs are not significantly changed. The scaling factor S is: 1 1 1 −1 −αR α h − h S = 1 + e BR i o . (5.4)

The broken exponential function has been further generalized by Comer´on et al. (2012) for cases where the disc has more than one break:

i=n r 1 1 − 1 − h Y αi−1,i(r−Ri−1,i) α h h Σ(r) = S Σ◦ e 1 1 + e i−1,i i−1 i . (5.5) i=2

The parameter n defines the number of exponential sections in the disc profile. Again, Σ◦ is the central surface brightness of the disc section inside the break radius, and α is the smoothness of the break, again fixed to a value of −1 α = 0.5 arcsec in papers I and IV. The disc scalelengths are now hi−1 for inside, and hi outside of the break radii (Ri−1,i). The scaling factor S is:

i=n 1 1 − 1 −1 Y −αi−1,i ri−1,i α h h S = 1 + e i−1,i i−1 i . (5.6) i=2

56 In paper III pipeline decompositions the edge-on galaxies discs were ﬁtted with a separate disc model: rx rx 2 rz Σ(rx, rz) = Σ◦ K1 sech . (5.7) hr hr hz

Here K1 is the modified Bessel function, rx and rz are the distances from the galaxy centre along, and perpendicular to the major axis. Similarly, hr and hz are the radial and vertical disc scalelengths, respectively. As shown by van der Kruit and Searle (1981a), this function corresponds to the line of sight integrated surface brightness distribution of a three dimensional disc, with an exponential radial and a sech2 vertical density distribution. The modified Ferrers function is used for modelling bars and lenses. It has a flat inner profile with a transition to zero surface brightness at a defined radius:  2−β α r Σ◦ 1 − , when r < rout Σ(r) = rout (5.8) 0, when r ≥ rout.

The parameter Σ◦ is the central surface brightness, α controls how sharp the transition to zero surface brightness near rout is, and β controls the slope in the inner parts of the profile. The Point Spread Function (PSF) is often used to model point sources, such as bright nuclei of galaxies. This is usually done using an Gaussian function, with a Full Width at Half-Maximum (FWHM) measured from the data, or by deriving a PSF image from the data. The PSF is also used to convolve the models in decompositions to match the data, which are blurred by seeing and telescope optics. The PSF of the S4G 3.6 µm images consists of a Gaussian core, and wide tails. Both components of the PSF are important for the retrieval of the correct bulge parameters in the two dimensional decompositions (paper III). However, the effect of the PSF on the disc parameters is negligible (paper III). Therefore, in the one dimensional fits of papers I and IV focusing on the outer discs, there is no need to use the PSF.

5.2 One dimensional methods

In one dimensional decomposition methods the structures of galaxies are studied using azimuthally averaged light proﬁles (see 3.1). This is also the greatest drawback in one dimensional methods, because information is lost about the ellipticity and position angle of the individual components. In- evitably, this leads to the question how valid such proﬁles are for quantitative study of galaxy structure, particularly for barred disc galaxies. Partially because of this limitation, most recent decomposition studies have moved to two dimensional methods, in which the ellipticity and position angle of individual

57 components can be accounted for. However, one dimensional methods are still useful for a number of reasons: the decompositions are easy to implement in many programming languages, the fit process is very fast, and in studies of roughly axisymmetric structures (e.g. elliptical galaxies, outer discs) the lost information about position angle and ellipticity is not as critical. Fur- thermore, surface brightness profiles obtained over elliptical isophotes fixed to the ellipticity and position angle of the outer disc are commonly used (see section 3.1). While it is possible to do one dimensional decompositions taking the whole galaxy into account (e.g. bulge+bar+disc), a simplified method is commonly used when studying the different disc profile types, which will be discussed below.

5.2.1 Marking the disc “Marking the disc” method is a simplified implementation of the one dimensional decomposition, in the special case that only disc component is used. In this approach the inner parts of the radial surface brightness profile, where bulges and bars dominate, are ignored. The radial profile functions (Eq. 5.2, 5.3, 5.5) are then fitted only to the regions of the galaxy where the disc light is the major contribution. This method leads to disc parameters that look correct (e.g. discussion in de Jong 1996), although bulges and bars can affect the disc parameters even at large radial distances. Therefore, the disc parameters retrieved through this method might not always agree with more detailed decompositions. Despite of the limitations, this method has been successfully used in the studies of the different disc profile types, as shown by Erwin et al. (2005), P06, E08, and G11. We also use this method in papers I and IV. A fundamental issue with this method is the uncertainty where to place the fit region. We have decided to set the outer limit where under- or overestimating the background sky by the standard deviation of the sky measurements (σsky) affects the radial surface brightness profile by an arbitrary amount, for example by ±0.33 mag arcsec−2 (i.e. paper IV, see examples in Fig. 5.1). Estimating the inner limit is not as straightforward. In papers I and IV we used bar size measurements from literature, and also visually made sure from the galaxy images that the inner limit is outside the bar radius (e.g. Fig. 5.1). However, in unbarred galaxies this criterion cannot be used. Furthermore, especially in the early-type unbarred galaxies with large bulges it is difficult to separate the bulge and disc contributions: the surface brightness profile of the whole galaxy has a smooth behaviour, which could be modelled with a single Sérsicfunction (see example in Fig. 5.2). For this reason, the earliest disc galaxy types (T = −3) were not included in the paper IV.

58 NGC4535 - log10 ( M* / MO • ) = 10.69 NGC4535 - Type II.i 16 rin rout 300 18 200 ] -2 20 100

0 22

-100 [AB mag arcsec 24 3.6 µ -200 26 -300 Bar radius 28 -300 -200 -100 0 100 200 300 0 50 100 150 200 250 Semi-major axis [arcsec]

NGC3346 - log10 ( M* / MO • ) = 10.16 NGC3346 - Type II 16 rin rout 100 18 ] 50 -2 20

0 22

[AB mag arcsec 24 -50 3.6 µ

26 -100

Bar radius RBR 28 -100 -50 0 50 100 0 20 40 60 80 100 120 Semi-major axis [arcsec]

NGC3642 - log10 ( M* / MO • ) = 10.40 NGC3642 - Type III 150 16 rin rout

100 18 ] 50 -2 20

0 22

-50 [AB mag arcsec 24 3.6 µ

-100 26

RBR -150 28 -150 -100 -50 0 50 100 150 0 20 40 60 80 100 120 Semi-major axis [arcsec]

Figure 5.1: Examples of the disc section fits for six galaxies. The fits are done between the inner (rin) and outer limits (rout), which are marked as solid orange lines in the surface brightness profiles and the galaxy images. The horizontal error bars indicate the region in which the fit region limits are altered when estimating the uncertainties. The fit is shown as a thick red line under the surface brightness profile, and the possible break radius (RBR) is indicated with purple long-dashed line. Bar radius is indicated with vertical blue dashed line in the surface brightness profiles.

59 NGC3684 - log10 ( M* / MO • ) = 10.18 NGC3684 - Type III 16 rin rout 100 18 ]

50 -2 20

0 22

[AB mag arcsec 24 -50 3.6 µ

26 -100 Bar radius RBR 28 -100 -50 0 50 100 0 20 40 60 80 100 Semi-major axis [arcsec]

NGC7171 - log10 ( M* / MO • ) = 10.58 NGC7171 - Type II 16 100 rin rout

50 ] -2 20

0 22

[AB mag arcsec 24 3.6

-50 µ

-100 Bar radius RBR 28 -100 -50 0 50 100 0 20 40 60 80 100 Semi-major axis [arcsec]

NGC1672 - log10 ( M* / MO • ) = 10.66 NGC1672 - Type II 16 rin rout 200 18 150 ]

100 -2 20 50

0 22

-50 [AB mag arcsec 24 3.6

-100 µ

-150 26 -200 Bar radius RBR 28 -200 -150 -100 -50 0 50 100 150 200 0 50 100 150 200 Semi-major axis [arcsec] Fig. 5.1 continued.

60 NGC4546 - log10 ( M* / MO • ) = 10.83 NGC4546 120 14 rout

16 80 ]

-2 18 40 20 0 22

-40 [AB mag arcsec 3.6

µ 24

-80 26

-120 28 -120 -80 -40 0 40 80 120 0 40 80 120 Semi-major axis [arcsec]

Figure 5.2: Example of a early-type (T = −3) galaxy in which the placement of the inner ﬁt limit is problematic, when using the “marking the disc” method. Shown is the Ks-band data from NIRS0S.

The formal statistical uncertainties resulting from fitting different functions to the data points are generally small. Rather, the placement of the fit region with the criteria discussed above is the largest source of uncertainty in the disc fit procedure. Therefore, in papers I and IV a Monte Carlo method is used to estimate the uncertainties in the different disc and break parameters caused by the fit region limits (see also Muñoz-Mateos et al. 2013). We draw 500 new values from uniform distribution for the inner and outer fit region limits. The distribution has a width of 0.15 × R24,IR, centred at the original user chosen radius. The 24 mag arcsec−2 radius of the infrared data (R24,IR) is chosen, so that the shallower NIRS0S data also reaches this radius. With the new fit region limits the fit is automatically redone, using the same initial values and user chosen profile type as in the original fit. In the end, the standard deviation of the returned fit parameters is calculated, representing the uncertainty of the original user supervised fit. In individual cases the parameters of the automatic fit can deviate a lot from the original fit (i.e. absolute difference ∆i = |xi − xori|, where ori refers to the original fit value), but on average over the 500 fits they are relatively small (median ∆i). In Fig. 5.3 we show, for Type II profiles in paper IV, the median xi and the median ∆i of the 500 automatic fits, as a function of the original value xori. It is noticeable that the inner disc scalelengths have the largest spread. This can be explained by the complex inner disc structures of some galaxies: when the inner fit limit is moved, bar and/or inner ring could be included in the fit region, which would change the slope dramatically. This can also be seen in the galaxies NGC 7171 and NGC 1672, shown in Fig. 5.1, in which galaxies the local slope of the surface brightness profile near the inner fit limit changes rapidly with radius.

61 Type II - inner disc µ0,i Type II - outer disc µ0,o ]

] 24 24 -2 -2

22 22

20 20

18 18 (mod) [mag arcsec (mod) [mag arcsec Type II - break radius RBR 0,i 0,o µ µ 16 16 Median 14 Median 14 10 14 16 18 20 22 24 14 16 18 20 22 24 -2 -2 Original µ0,i [mag arcsec ] Original µ0,o [mag arcsec ] (mod) [kpc]

Type II - inner disc hi Type II - outer disc ho BR

Median R 1

10 10

1 10 (mod) [kpc] (mod) [kpc] i o Original RBR [kpc] Median h 1 Median h 1

1 10 1 10 Original hi [kpc] Original ho [kpc]

Figure 5.3: Demonstration of how the change in the fit region limits affects the parameters of Type II profiles retrieved from the disc profile fits of the infrared data. Here the median values (red dots) of the profile parameters calculated from the 500 automatic fits with varying inner- and outer fit region limits, are shown as a function of the original fit value. The error bars show the median absolute difference of the automatic fits relative to the original fit value (median[|xi − xori|]). Shown separately are the central surface brightness and scalelength of the inner (leftmost panels) and outer disc sections (middle panels), and the break radius (right panel).

Nevertheless, by altering the fit region limits the uncertainty of placing the inner limit, and the uncertainty of the background estimate, are both taken into account. Generally, the fit procedure is robust, and for example the relative absolute difference (median[∆i/xori]) is on average ∼ 6% for the Type II break inner disc scalelength. For the Type I and III profiles the changes in the fit parameters are smaller compared to Type II profiles.

62 Figure 5.4: Two dimensional decomposition result of the galaxy NGC 4314 from the S4G pipeline 4. The decomposition is presented here using the GALFIDL routines2, in which every dot represents individual pixel in the model and data images.

5.3 Two dimensional methods

As already mentioned, the greatest advantage of two dimensional decomposition methods is the ability for the different components to have different ellipticities and position angles. In practice, in two dimensional decompositions the one dimensional radial surface brightness profile functions presented in section 5.1.1, are expressed in two dimensions by using the isophotal co- ordinates of generalized ellipses (Athanassoula et al. 1990):

1 ! c +2 y − y C0+2 0 C0+2 0 r(x, y) = |x − x0| + . (5.9) q

The parameter C0 controls the shape of the ellipse: with C0 = 0 the shapes are pure ellipses, when Co < 0 the isophotes become more disky, and more boxy when C0 > 0. Parameter q is the axial ratio (b/a) and (x0, y0) is the

63 Figure 5.5: Presentation of the data image (upper left panel), masked data image (upper right), decomposition model (lower left), and the residual image (data-model, lower right) of galaxy NGC 4314, from the pipeline 4 decompositions of the S4G galaxies. centre of the isophote. The pipeline decompositions of paper III used GALFIT software (Peng et al. 2002, 2010), and the decompositions are visualized using GALFIDL routines2. Examples of paper III pipeline decomposition are shown in Figs. 5.4 and 5.5. NGC 4314 is an example in which the four different components were fitted in the pipeline decompositions: nucleus (modelled with a PSF), bulge (Sérsicmodel), bar (Ferrer model), and disc (exponential disc model). Particularly useful method for estimating the two dimensional decomposition results is the residual image (Fig. 5.5 lower right panel), in which the structures of the galaxy that were not captured by the decomposition model are highlighted. In this case, the prominent outer ring structure, which is not modelled in the decompositions, is well visible in the residual image.

2http://sun3.oulu.fi/~hsalo/galfidl.html

64 5.3.1 Disc breaks The disc breaks have been mainly studied in the radial surface brightness profiles, using one dimensional methods. This is despite the fact that most of the major two dimensional decompositions software provide the ability to fit disc breaks (e.g. GALFIT (Peng et al. 2002, 2010), BDBAR (Laurikainen et al. 2005), BUDDA (de Souza et al. 2004; Gadotti 2008; Kim et al. 2014), and IMFIT (Erwin 2015)). Only a few studies have examined the disc breaks using two dimensional decompositions so far (i.e. Kim et al. 2014; Head et al. 2015; Ruiz-Lara et al. 2016). The two dimensional pipeline decompositions in paper III do not include disc breaks, and instead a single exponential disc was used in a majority of disc galaxies. In some cases two disc components were used, when it was obvious that the galaxy has two embedded discs (i.e. DD models in paper III). Thus the paper III pipeline decompositions provide an opportunity to compare the disc properties derived from one dimensional methods with disc breaks from papers I and IV, with the two dimensional decompositions using only one disc component. In paper III this was done with paper I data, and now in Fig. 5.6 we expand the Fig. 29 of paper III to also include data from paper IV. For single exponential Type I galaxies (Fig. 5.6 upper panel) the agreement is very good. In galaxies with a break (Fig. 5.6 middle and lower panels) the single disc scalelength obtained using two dimensional methods falls between the inner and outer disc scalelengths measured using one dimensional methods. Thus, the two dimensional decompositions provide a good estimate of the overall scalelength of the disc, even when no break is fitted. Two dimensional decompositions of Kim et al. (2014) also fit the disc breaks. In addition, they compared their results of Type II breaks with those galaxies that were also classified as Type II in Muñoz-Mateoset al. (2013), who used one dimensional methods, similar to ours in papers I and IV. Both Kim et al. and Muñoz-Mateoset al. used the same 3.6 µm data, which we have also extensively used. Kim et al. found that the break radius is similar in the one and two dimensional methods. However, the comparison sample included only 32 galaxies, and of only Type II profiles. Further studies utilizing one and two dimensional decompositions with disc breaks are needed to see if the results are indeed comparable.

65 Type I 1D (Papers I and IV) 2D (Paper III)

10 h [kpc]

8 9 10 11 log10 ( M* / MO • ) Type II 1D inner (Papers I and IV) 1D outer (Papers I and IV) 2D (Paper III) 10 h [kpc]

8 9 10 11 log10 ( M* / MO • ) Type III 1D inner (Papers I and IV) 1D outer (Papers I and IV) 2D (Paper III) 10 h [kpc]

8 9 10 11 log10 ( M* / MO • )

Figure 5.6: The disc scalelengths derived from one and two dimensional methods as a function of the galaxy stellar mass. Shown separately, based on the radial surface brightness profile classification, are the single exponential discs (upper panel), Type II down-bending break galaxies (middle panel), and Type III up-bending break galaxies (lower panel). The lines in each panel indicate linear fits to the data points, made with least absolute deviation method.

66 Chapter 6

Results

6.1 Disc proﬁle type fractions

In paper IV we present the fractions of the different disc profile types in a large range of morphological types and galaxy stellar masses (see Table 6.1 leftmost column, and Fig. 6.1). The most common disc profile type is the down-bending Type II, which are found in 45 ± 2% of the galaxies1. The Type II profiles are also evenly spread among the morphological types, and are present in ∼ 40 − 50% of the galaxies in all Hubble type bins of Fig. 6.1, left panel. The single exponential Type I profiles are found in 31 ± 2%, with the fraction increasing towards early- and late-type galaxies from a minimum at T = 2. The up-bending Type III profiles are the least common, found only in 23 ± 2% of the galaxies, and are most common in T . 2 galaxies. The Type II.i profiles, in which the break is seen inside the bar radius, are rare being found in 6 ± 1% of the galaxies. We separate the II.i breaks from Type I and Type II profiles, because the Type II.i could be included in either class. Galaxies with two breaks are also rare, and are found in 41 galaxies out of the 759 in paper IV (∼ 5% of all galaxies). The overall fractions of the different disc profile types are the same, within uncertainties, as found in paper I (Type I 32 ± 3%, Type II.i 7 ± 2%, Type II 42 ± 3%, Type III 21±2%), even though the sample of paper I contained only high mass galaxies from a narrower morphological type range (−3 ≤ T ≤ 7). In Table 6.1 we show separately the profile type fractions in barred and unbarred galaxies. It appears that Type I and II fractions are not significantly different in these galaxies. However, Type III fraction in unbarred galaxies is twice of that in barred galaxies. Understandably, compared to the global bar fraction of ∼ 2/3 (see section 2.4.2), the bar fraction of Type III is lower. The fractions of the disc profile types are affected by the galaxy stellar mass. In Fig. 6.1 (right panel) we show that the Type I profiles are less

1The uncertainties of the proﬁle type fractions are calculated using binomial statistics, and denote the ±1σ uncertainties.

67 Morphological types Stellar masses 0.8 0.8 Type I Type I Type II.i Type II.i Type II Type II 0.6 Type III 0.6 Type III

f 0.4 f 0.4

0.2 0.2

0.0 0.0 -2 0 2 4 6 8 10 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 Hubble type log10 (M*/MO •)

Figure 6.1: The fractions of disc profile types in the paper IV infrared data, shown as a function of mid-IR galaxy morphological type, and galaxy stellar mass. The error bars are calculated using binomial statistics, and denote the ±1σ uncertainties. In the right panel the vertical grey dashed lines indicate the stellar mass bin limits, selected so that the bins have roughly equal number of galaxies. common in high mass galaxies, while the fractions of Type II and Type III breaks increase with stellar mass. The increase in Type III fraction with stellar mass can be attributed to the fact that these profiles are more common in the T . 4 galaxies, which are generally more massive (see for example Fig. 4.1). The fraction of Type II.i breaks also increases marginally with stellar mass, possibly due to bars being more pronounced in the surface brightness profiles of massive galaxies. In section 6.2 we discuss in more detail possible causes for Type II and Type I dependencies on Hubble type and stellar mass. In paper IV it was also examined if the profile type changes because of the observed wavelength. We found that the fractions of the disc profile types are the same, within uncertainties, regardless of the wavelength used, if the data has similar depth. Out of the 302 galaxies, in which the profiles could be studied reliably in all six wavelength bands (i.e. in u, g, r, i, z, and Ks or 3.6 µm bands), 285 (∼ 95%) had the same profile type in all wavelengths. Herrmann et al. (2013) studied 141 late-type dwarf galaxies using a large variety of wavelengths ranging from far ultraviolet to 4.5 µm mid-infrared. They found that in 91% of their sample galaxies the profile type was the same in all the different bands.

6.1.1 Comparison with other studies In Table 6.2 we compare the disc proﬁle type fractions of paper IV to other studies performed at low redshift, namely in G11 (includes also statistics

68 Paper IV (N=759) Barred (N=530) Unbarred (N=229) Type Fraction Bar fraction Type Fraction Type Fraction

I 31 ± 2% (237) 76 ± 3% I 32 ± 2% (169) I 30 ± 3% (68) II.i 6 ± 1% (46) 100 ± 0% II.i 9 ± 1% (46) II.i 0 ± 0% (0) II 45 ± 2% (339) 71 ± 2% II 46 ± 2% (244) II 41 ± 3% (95) III 23 ± 2% (179) 55 ± 4% III 18 ± 2% (93) III 37 ± 3% (85)

Table 6.1: Statistics of the disc profile types of the infrared data used in paper IV. Shown separately are the bar fraction for the different profile types, and the profile type fractions for barred and unbarred galaxies. The numbers in parenthesis correspond to the number of galaxies. The errors are calculated using binomial statistics, and denote the ±1σ uncertainties. Some galaxies have two breaks in their discs (e.g. Type II + III) and are counted twice in the statistics, explaining why the overall percentages can be above 100%.

from P06 and E08 galaxies), Muñoz-Mateoset al. (2013), and Herrmann et al. (2013) (see also Fig. 4.3). Given that the break type fraction depends on stellar mass and galaxy morphology, exact agreement between the different samples is not expected. Muñoz-Mateoset al. (2013) (hereafter M13) and G11 study mainly high mass galaxies, from a similar morphological type range as paper IV. The methods to study the disc profiles are the same in these three studies, and also the disc profile type fractions in all of them agree with each other quite well. Particularly Type II fractions are nearly the same among the three studies, as are the Type III fraction in paper IV and in M13. However, in paper IV we find more Type I profiles (31 ± 2%, 21 ± 3% in G11, and 20 ± 3% in M13), and the fraction of Type III profiles in paper IV is also significantly smaller than in G11 (23 ± 2% vs. 38 ± 4% in G11). M13 finds considerably more Type II.i profiles than G11 or paper IV. These differences in the disc profile type fractions could arise from two factors: differences caused by the used galaxy sample and data, or subjective differences in the disc profile classification. The sample properties are the main reason for the larger fraction of Type I profiles in paper IV, compared to M13 and G11, because also low mass galaxies are included. However, the data used in the different studies is not an important factor. M13 uses the same 3.6 µm data as paper IV, whereas G11 uses SDSS or independent observations reaching similar image depth. On average the SDSS data has a depth comparable to the 3.6 µm images, and thus the profile type should be unaffected by the observed wavelength and the data used in these studies (see also paper IV, or previous section 6.1). To address the differences further, we compare the profile type classifica-

69 Paper IV Guti´errezet al. (2011) Mu˜noz-Mateoset al. (2013) Herrmann et al. (2013) 3.6 µm (N=759) g, r, R, B-bands (N=183) 3.6 µm (N=218) FUV-4.5 µm (N=141) −2 ≤ T ≤ 9 −3 ≤ T < 8.49 −3 ≤ T ≤ 7 dIm, Sm, and BCD galaxies

Type Fraction Type Fraction Type Fraction Type Fraction

I 31 ± 2% I 21 ± 3% I 20 ± 3% I 8% II.i 6 ± 1% II.i 19 ± 3% II 45 ± 2% II 53 ± 3% II 67% II.i + II 51 ± 2% II.i + II 50 ± 3% II.i + II 72 ± 3% III 23 ± 2% III 38 ± 4% III 19 ± 3% III 22% 70

Table 6.2: Statistics of the disc profile types in the paper IV infrared data compared with studies in the literature. We give the used wavelength in each study, the morphological type range of the sample galaxies, and the number of galaxies in parenthesis. Herrmann et al. (2013) used multitude of different wavelengths, ranging from far ultraviolet (FUV) to 4.5 µm infrared wavelengths, and the reported statistics are for galaxies where they found the break type to be the same in all their observed wavelengths. Profile type fractions of Muñoz-Mateos et al. (2013) were calculated using the tables provided in their study. In paper IV and Muñoz-Mateoset al. (2013) the uncertainties denote the ±1σ uncertainties, calculated using binomial statistics. Some galaxies show two breaks in their discs and are counted twice, explaining why the overall percentages can be above 100%. III 2 0 1 16 III 7 1 2 15

II 8 139 1 II 7 1061 0 Paper IV Paper IV II.i 43 3 0 II.i 08 0 0

I 10 1 10 12 I 16 2 13 3

I II.i II III I II.i II III Gutierrez et al. 2011 Munoz-Mateos et al. 2013

Figure 6.2: Illustration of how the profile type classifications are distributed for galaxies that are included in papers IV and Gutiérrezet al. (2011) (left panel), and papers IV and Muñoz-Mateoset al. (2013) (right panel). Note that galaxies in which additional outer break was found by either study are not counted here. tions of galaxies which are included in papers IV and M13 (152 galaxies), or in paper IV and P06, E08, and G11 (118 galaxies). Surprisingly, less than two thirds of the galaxies have exactly the same disc profile type in the different studies: 98 galaxies (64%) in papers IV and M13, and 68 galaxies (∼ 58%) in papers IV, P06, E08, and G11. In some galaxies in which the break type agrees, an additional outer break was found by another study (13 galaxies of papers IV vs. M13, and 9 of papers IV vs. P06, E08, G11). We summarize the differences of the profile type classifications in Fig. 6.2. It is notable that the most common difference is that galaxy is classified as Type I in one study, while a break is seen in the other. Particularly, profiles classified as Type III in G11 are more often Type I in paper IV, than vice versa. These 12 Type III breaks appear at particularly low surface brightness levels in P06, E08, −2 and G11, with a median of µbreak ∼ 25.3 mag arcsec in the r-band. This is close to the limiting surface brightness of the infrared and optical data in paper IV2, and explains the tendency to classify them as Type I in paper IV. Furthermore, Type II profiles in paper IV are often classified as Type II.i in M13. In paper IV we have used bar size measurements from Herrera- Endoqui et al. (2015), while M13 uses their own measurements. This can explain the discrepancy, and possibly highlights the particular cases where the uncertainties in the bar detection and size measurements are large.

2 −2 Paper IV Table 2: 3.6 µm median µlimit = 25.24 ± 0.43 mag arcsec , and r-band −2 median µlimit = 25.7 ± 0.48 mag arcsec

71 Herrmann et al. (2013) studied the disc profiles of late-type dwarf galaxies (i.e. T = 9−10), including also a small number of blue compact dwarfs. They used mostly optical U, B, and V band images, but included also observations ranging far ultraviolet to 4.5 µm infrared wavelengths. In Table 6.2 we show their statistics, reported for galaxies in which the profile type is the same in all of the observed bands. Their Type III fractions agrees with paper IV and M13, but Herrmann et al. (2013) found a very low fraction of Type I, and a high fraction of Type II profiles (∼ 8% and ∼ 67%, respectively). The fractions obtained in paper IV in the lowest mass bin (i.e. log10(M∗/M ) . 9.5, see Fig. 6.1) for Types I and II do not agree with those of Herrmann et al. (2013). As discussed by Herrmann et al. (2013) it is challenging to study the low mass late-type dwarfs. Namely, Type II down-bending break could manifest in the surface brightness profile due to several reasons, for example: the galaxy centre is not always obvious, the galaxies have clumpy star forming morphology, and the galaxies can be globally asymmetric. The infrared data is less affected by star forming morphology, so that the profile could be a single exponential even when a break is seen in the optical. Our results of dwarf T = 9 − 10 galaxies in the S4G at 3.6 µm show that they are mostly single exponential Type I galaxies (paper IV). However, further study is needed to disentangle differences between our results and those of Herrmann et al. (2013).

6.2 Connections with galaxy structure components

Motivated by the division of Type II profiles to subclasses II-OLR and II-CT by P06, E08, and G11, we have connected the Type II break location visually to the different morphological structures of galaxies in papers I and IV. In Fig. 6.3 we show the distribution of Type II profiles associated with the different structures, among galaxy morphological types and stellar masses. Two major groups of Type II breaks are identified: breaks connected with bar resonances (i.e. outer rings and lenses), and breaks connected with possible star formation threshold (i.e. visual spiral outer edge). These two major groups are directly comparable to the original Type II-OLR and II-CT by P06 and E08. Here we include into the outer rings all the different outer ring morphologies (i.e. rings, pseudo-rings, and ring-lenses; more details for example in Comerónet al. 2014). Some of the Type II breaks were also connected with inner rings in unbarred galaxies, bright star formation regions inside the spiral arms, or no connection was found with the galaxy structure. P06 showed that Type II-OLR breaks are more common in the early- type galaxies (i.e. T ≤ 4). This is true for Type II breaks that we have connected with the outer rings and lenses, which mostly appear in T . 3

72 Type II break connection Type II break connection 60 60 All type II All type II inner rings inner rings 50 outer rings 50 outer rings outer lenses outer lenses inside spiral structure inside spiral structure 40 visual spiral outer edge 40 visual spiral outer edge No connection No connection

N 30 N 30

20 20

10 10

0 0 -5 0 5 10 8 9 10 11 Hubble T-type log10 (M*/MO •)

Figure 6.3: The distribution of the Type II breaks from papers I and IV connected with diﬀerent galaxy structure components among mid-IR morphological types (left panel), and stellar masses (right panel).

Type II ring and lens connection Type II spiral connection 30 30 outer ring visual spiral outer edge inner ring inside spiral structure outer lens 25 25

20 20

15 15 (IR) [kpc] (IR) [kpc] BR BR R 10 R 10

5 5

0 0 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Feature radius [kpc] Spiral max radius [kpc]

Figure 6.4: Comparison between the radius of the galaxy structure associated with Type II breaks, and the break radius. In the left panel we show the breaks associated with resonance structures, and in the right panel with spiral arms. In both panels the dashed line indicates where the break radius equals the structure radius, and in right panel the solid line indicates break radius at 0.5× the structure radius.

73 galaxies. In total, these breaks consist of ∼ 38% of all Type II breaks in the papers I and IV. The early-type galaxies are generally also more massive, and the fraction of Type II associated with outer rings rises with increasing galaxy stellar mass (Fig. 6.3 right panel). Type II breaks associated with the visual spiral outer edge do not show a strong dependency on the galaxy stellar mass. However, these breaks are more common in late-type galaxies (T & 4), as also found by P06. The breaks associated with the spiral outer edges consist of ∼ 47% of all Type II breaks in our study. Only ∼ 10% of Type II breaks are thus found to be connected with galaxy structures other than outer rings/lenses or visual spiral outer edge. The decrease of Type II profiles towards lower mass galaxies could be a manifestation of the decreasing outer ring and lens fraction, with decreasing galaxy stellar mass (e.g. Comerónet al. 2014; Herrera-Endoqui et al. 2015). This mass dependency of Type II breaks associated with bar resonance structures, provides an explanation for the increasing fraction of single exponential discs with decreasing galaxy stellar mass. When this is combined with the decrease of Type III fraction towards lower galaxy masses, the increase of Type I fraction with decreasing stellar mass is accounted for. Moreover, the fraction of Type I profiles in Fig. 6.1, left panel, follows the distribution of the outer rings and lenses among the morphological types, but in inverted fashion (compare to Fig. 9 in Herrera-Endoqui et al. 2015). The outer ring and lens structures are most commonly found in T ≈ 2 galaxies, at which types the single exponential Type I profiles are least common. Using the measurements of galaxy structures in the literature (i.e. Lau- rikainen et al. 2011; Comerónet al. 2014; Herrera-Endoqui et al. 2015), we have compared how the break radius agrees with the radius of the structure it is associated with. In Fig. 6.4, left panel, we show that the radius of the outer rings and lenses associated with Type II breaks agree well with the break radius (see also Erwin et al. 2008b). This solidly proofs the connection of these structures with the Type II breaks, and that the first visually made association also agrees well with independent measurements. The spiral outer radius agrees well with the break radius of Type II associated with the spiral outer edge, when compared to the spiral outer radius obtained from logarithmic spiral section fits of Herrera-Endoqui et al. (2015) (Fig. 6.4 right panel). Furthermore, those Type II breaks that are visually associated with bright star formation inside the spirals, have a break radius of about half of the spiral maximum radius. For the Type III breaks we have not found any clear connection with the structures components. In paper I we found that approximately 1/3 of the Type III profiles in bright galaxies might be associated with inner/outer rings and lenses. Our samples also consist mostly of relatively face-on galaxies, for which the separation to III-d and III-s by the behaviour of the outer disc ellipticity (Erwin et al. 2005) is not reliable.

74 6.3 Break properties

In papers I and IV we have examined the general parameters of the discs and their breaks in the infrared. These parameters are the break radius (RBR), inclination corrected surface brightness at the break radius (µBR, 3.6µm), the location of the break as measured in the inner disc scalelengths (RBR/hi), and the break strength measured by the inner/outer disc scalelength ratio (hi/ho). The only parameter that correlates with galaxy mass is the break radius, which increases with galaxy size, measured at 25.5 mag arcsec−2 radius (see also Azzollini et al. 2008b; Muñoz-Mateoset al. 2013). The values of µBR, 3.6µm, RBR/hi, and hi/ho have large scatter and do not significantly change with the galaxy stellar mass (3.6 µm average values shown in Table 6.3). These results have two important implications. First, the breaks are detectable in low mass galaxies in the same manner as in high mass galaxies, due to the similar surface brightness levels at the break radius and break strength. Second, different processes forming the breaks modify the old stellar disc, probed by the infrared data, in a similar manner in a large range of galaxy morphologies and stellar masses.

The mean value of µBR, 3.6µm for the Type II breaks is 23.3 ± 1.1 mag −2 7.4±0.4 −2 arcsec , which corresponds to a stellar surface density of 10 M kpc . This value does not significantly change when calculated separately for the two major classes of Type II profiles: those associated with outer rings/lenses, and those with visual outer edge of the spiral structure. In the simulations of Type II formation due to star formation threshold, the predicted break 7 8 −2 location in z = 0 galaxies is at ∼ 10 − 10 M kpc (Roˇskar et al. 2008b; Sánchez-Blázquezet al. 2009; Foyle et al. 2008). This agrees excellently with the observed result. The two other parameters characterising the breaks, hi/ho and RBR/hi, do change slightly when separated to the two major classes of Type II breaks. On average, the breaks associated with outer rings or lenses are slightly more inside the galaxy as measured by RBR/hi, and are more prominent as indicated by hi/ho, than the breaks associated with the visual outer edge of the spiral structure. Regardless of this association, these values agree well with previous results reported in observational and simulation studies (e.g. P06, Foyle et al. 2008; Muñoz-Mateoset al. 2013). Dedicated simulation studies in which the Type II break manifests through formation of an outer ring or lens, have not been presented in the literature. However, as indicated by the association with different structures and the slightly different properties of the breaks, such simulations would be helpful to further constrain the break formation mechanisms. For Type III profiles, very few simulation studies exist which focus on their formation processes, and they provide only a few parameters to compare with observations. Minor merger simulations of Younger et al. (2007) found that RBR/hi = 3−4, consistent with previous observational studies and with

75 Type II Type II Type II Type III Parameter outer rings/lenses spiral outer edge

µBR, 3.6µm 23.3 ± 1.1 23.4 ± 1.1 23.5 ± 1.1 23.9 ± 1.0 RBR/hi 1.8 ± 0.9 1.8 ± 0.8 2.2 ± 0.8 4.1 ± 1.2 hi/ho 2.3 ± 1.3 2.9 ± 1.8 1.9 ± 0.7 0.5 ± 0.1 Table 6.3: Mean values, and ±1σ uncertainties, of the parameters commonly used to characterise the break properties derived from the 3.6 µm data. our result of 4.1±1.2. In major merger simulations resulting in the formation of S0 galaxies, Borlaff et al. (2014) found that the values of the R-band µBR in their simulated Type III break galaxies are similar to the observational −2 results (µBR ≈ 22 − 27 mag arcsec ). Borlaff et al. (2014) also give three linear scaling relations based on their simulations, which link the inner (hi) and outer disc (ho) scalelengths, and the R-band surface brightness at the break radius (µBR), to the break radius (RBR). Using the data from paper IV, we have tried to reproduce these relations from the SDSS r-band data of Type III profiles in S0 galaxies (i.e. T ≤ 0): Borlaff et al. (2014) paper IV

hi = 0.22RBR + 0.20 hi = 0.23RBR + 0.29 ho = 0.83RBR − 1.9 ho = 0.40RBR + 0.79 µBR = 0.22RBR + 21.84 µBR = 0.13RBR + 20.16.

The best agreement is obtained for the hi-RBR relation, but otherwise little similarities are found. To conclude, some of the Type III break parameters returned from simulations agree with observations, but the problem remains poorly constrained. The fact that the properties of Type III breaks do not seem to be correlated with galaxy mass should restrict the properties of Type III formation processes. Especially if the Type III breaks originate from galaxy encounters and merging events, in which case the orbital properties, mass ratios, and gas content of the progenitor galaxies all aﬀect the properties of the remnant galaxy.

6.4 Changing properties with wavelength of observations

In paper IV we use SDSS data in five different optical wavelengths, in addition to the 3.6 µm or Ks-band infrared data. We derive the properties of the different disc profile types in all the six wavelength bands. Shorter wavelengths, especially the SDSS u-band, trace the hot youngest stars, while in

76 the longer infrared wavelengths older stellar populations dominate. There- fore, comparison between the different wavelengths gives a rough estimate of stellar age effects on the parameters of the disc profiles. We found that the scalelengths (h) are mostly affected by the observed wavelength. In Fig. 6.5 we show the behaviour of the h normalized with the values of the Ks or 3.6 µm infrared data (h/h(IR)). In Type I profiles h increases systematically with shorter wavelengths (Fig. 6.5 upmost panel). The effect is strongest in the u-band, in which the h is on average a factor ∼ 1.2 larger than in the infrared. Weak morphological dependency is also found for all the five SDSS bands: h/h(IR) increases slightly from the early-type galaxies until T ≈ 5, beyond which it decreases. The gas-rich late-type (T & 5) galaxies generally show signs of prominent recent star formation. The star forming regions are also visible in the 3.6 µm data, due to a contribution from PAH-elements and hot dust associated with star formation (Meidt et al. 2012), which possibly explains the decrease in h/h(IR) in the latest galaxy types. Disc scalelengths have been found to depend on the observed wavelength also in some previous studies (e.g de Jong 1996; MacArthur et al. 2003; Fathi et al. 2010). For example, Fathi et al. (2010) studied 30 000 spiral galaxies (1 ≤ T ≤ 8), and found that the average h has an absolute increase of 35% from SDSS r to u-band. However, in Fig. 6.5 we do not see such large changes, which we suspect to be due Fathi et al. (2010) using only single exponential disc profile.

Moving to the Type II profiles, the inner disc hi are a factor of ∼ 2.5 larger in the u-band than in the infrared (Fig. 6.5 middle left panel). This increase in hi indicates a presence of young stellar populations in the inner disc, which flattens the surface brightness profile at shorter wavelengths. There is no sign of morphological dependency on hi/hi(IR), which is intriguing when the Type II profiles are associated with different structures in early- and late-type galaxies. However, in addition to the spiral structure, the outer rings are also places of prominent star formation. Even in the S0s star formation is seen in the outer rings through UV-observations (e.g. Salim et al. 2012). Moreover, break associated with star formation threshold in late-type galaxies would naturally restrict most young stars to the inner disc.

We also find that the outer disc ho decreases 10% from infrared to u- band in late-type galaxies with Type II profiles (T & 4, Fig. 6.5 middle right panel). However, in early-type galaxies (T . 4) no difference is found. This separation occurs at the same morphological type where the division of Type II breaks, associated with outer rings and spiral outer edge, takes places (see Fig. 6.3). Therefore, it seems that in T . 4 galaxies ho is affected by the angular momentum redistribution imposed by the bar, independent of stellar age. In T & 4 galaxies the Type II break is associated with star formation threshold, and the outer disc is populated by radially migrated older stars increasing ho in the infrared, as suggested also by the simulation models by

77 Roˇskar et al. (2008b). The behaviour of hi and ho indicates also that parameters commonly used to characterise the breaks, hi/ho or RBR/hi, depend on the observed wavelength (see also Radburn-Smith et al. 2012; Bakos and Trujillo 2012). These parameters are commonly used to compare observations with simulations, so one needs to be sure that the values derived from similar stellar populations are compared. Furthermore, in T & 4 galaxies with Type II breaks the behaviour of hi and ho is consistent with the U-shaped colour (and age) profiles observed for number of galaxies with a Type II profile (Azzollini et al. 2008b; Bakos et al. 2008; Yoachim et al. 2010, 2012; Roediger et al. 2012; Zheng et al. 2015). Our result also indicate that in T . 4 galaxies the colour and age profiles do not show as prominent U-shape, which has been found to be the case in some galaxies with a Type II break (e.g. Ruiz-Lara et al. 2016). Further study is needed to examine if the galaxy morphology, or the structure component associated with the Type II break, affects the shape of the colour and age profiles. Detailed analysis of colour profiles of the galaxies was out of scope in paper IV, but will be examined in a future study. For the Type III profiles we find that hi increases slightly with decreasing observed wavelength (Fig. 6.5 lower left panel). However, in T & 0 galaxies ho is on average a factor ∼ 1.15 larger in the u-band, compared to infrared (Fig. 6.5 lower right panel). This increase in ho from infrared to u-band is not surprising for spiral galaxies, because the spiral structure and star formation continue beyond the break. The result for the T & 0 galaxies is also compatible with a mechanism that boosts star formation in the outer discs, creating the observed up-bending in the surface brightness profile. One possible mechanism is the extended UV-discs (Gil de Paz et al. 2005; Lemonias et al. 2011), but systematic connection between the extended UV-discs and Type III profiles has not yet been presented in the literature. In the T . 0 early-type galaxies the outer disc scalelength does not change with observed wavelength, and thus the formation mechanism of the up-bend could also be different.

78 Type I 1.4 2.24±0.16 2.83±0.17 2.32±0.18 1.88±0.12 u / IR g / IR 1.3 r / IR i / IR z / IR 1.2

h / (IR) 1.1

1.0

0.9 -2 0 2 4 6 8 10 Hubble type Type II - inner disc Type II - outer disc 4.0 5.09±0.83 4.30±0.41 3.41±0.26 2.88±0.19 1.15 2.13±0.22 2.18±0.14 1.78±0.14 1.54±0.13 u / IR 3.5 g / IR 1.10 r / IR i / IR 1.05 3.0 z / IR

1.00 (IR)

(IR) 2.5 o i / h / h i

o 0.95 h 2.0 h

0.90 u / IR 1.5 g / IR 0.85 r / IR i / IR 1.0 0.80 z / IR -2 0 2 4 6 8 10 -2 0 2 4 6 8 10 Hubble type Hubble type Type III - inner disc Type III - outer disc 1.4 1.21±0.15 2.03±0.36 1.61±0.11 1.12±0.19 1.4 2.37±0.30 3.32±0.54 3.10±0.31 1.97±0.36 u / IR u / IR g / IR g / IR 1.3 r / IR 1.3 r / IR i / IR i / IR z / IR z / IR 1.2 1.2 (IR) (IR) o i / h / h 1.1 1.1 i o h h

1.0 1.0

0.9 0.9

-2 0 2 4 6 8 10 -2 0 2 4 6 8 10 Hubble type Hubble type

Figure 6.5: The mean behaviour of the disc scalelengths in the diﬀerent SDSS bands normalized by the infrared measurements, as a function of the galaxy mid-IR morphological type. Shown separately for the Type I (upper panel), Type II inner and outer disc sections (middle panels), and Type III inner and outer disc sections (lower panels). The bins are selected so that they include similar number of galaxies. Value on top of each bin shows the mean and ±1σ uncertainty of the infrared measurements. The error bars indicate the standard error of the mean.

79 80 Chapter 7

Conclusions and future work

The discs of galaxies are constantly evolving structures, with the evolution driven by large variety of internal and external processes. Deviations from single exponential behaviour of the disc surface brightness decline, disc breaks, are though to have formed through these evolutionary processes. In this thesis we have presented the overall fractions of the different disc profile types using the infrared data. The sample we use is the largest used so far, and also covers the widest range of galaxy stellar masses. We found that the most common profile type is the Type II down-bending break (found in 45 ± 2% of the galaxies), as also shown in the previous studies. The single exponential Type I profiles are the second most common (31 ± 2%), while the up-bending Type III profiles are the least common (23 ± 2%). Our new finding is that the break type fractions depend on galaxy stellar mass, with the fraction of Type I profiles decreasing, and the fractions of Types II and III increasing with galaxy stellar mass. This stellar mass dependency is explained by two factors: the Type III profiles are more common in early-type galaxies which are generally more massive, and the fraction of Type II breaks associated with outer rings/lenses, which also increases with galaxy stellar mass in a manner consistent with the occurrence rate of such structures. The galaxy stellar mass affecting the different profile type fractions can also explain some of the differences between studies. However, we have also shown that there are subjective differences in the break type classification between studies: even in 30% of the galaxies a break is found in a galaxy which is classified as single exponential in some other study. Some of the differences in the classifications can be explained by the used data. However, no clear consensus has been established on how large deviations in the surface brightness profile can be considered as a disc break, so that subjectivity remains to be one of the problems in the disc break studies. Nevertheless, it has been clearly established among several studies that most galaxies have either down- or up-bending breaks. It remains an open question why some galaxies have a single exponential disc, and what ultimately drives the global

81 fractions of the disc breaks. We also confirm previous results of Type II properties using the infrared data: Type II breaks are mainly connected with outer rings in the early-type galaxies (e.g. II-OLR breaks, T . 4, ∼ 38% of all Type II), and with outer edges of the spiral structures in late-type galaxies (e.g. II-CT breaks, T & 4, ∼ 47% of all Type II). This separation also implies a different formation mechanism for these two classes. In the early-type galaxies bars are possibly more effective in redistributing angular momentum in the disc, and causing a subsequent formation of resonance structures. On the other hand, in the late-type galaxies the star formation threshold appears to dominate the location of the break, as very little star formation is usually present beyond the spiral structure in the outer disc. Nevertheless, star formation seems to be connected with the inner disc section of the Type II, regardless of the formation mechanism, as seen in the changing value of the inner disc scalelength (hi) with the observed wavelength. Furthermore, in the multi-wavelength data of T & 4 galaxies we find possible evidence of stellar radial migration populating the outer disc with old stars. This is seen through the slight increase of the outer disc section scalelength (ho) with longer observed wavelengths. Similar behaviour is not seen in ho of T . 4 galaxies, which could explain why some Type II profiles do not have the characteristic U-shaped colour and age profile. Precise information about the stellar populations will be crucial to further constrain the properties of the Type II profiles associated with different galaxy structures. So far integral field spectroscopy has been mainly used to study late-type spiral galaxies with Type II profiles, but early-type galaxies with Type II breaks associated with outer rings have not received much attention. The formation processes of the up-bending Type III profiles remain poorly understood. The proposed formation mechanisms of Type III profiles in simulations include either major- or minor mergers. Some support to these models is also found in observational studies through a correlation between the properties of Type III profiles, and the environmental properties (e.g. Type III disc scalelength and Dahari-parameter correlation in paper I), but these observational results are not conclusive. Majority of Type III profiles are found in the early-type spirals and S0s (i.e. T < 2). In principle, it is possible that an environmentally driven transformation of a spiral into S0 could simultaneously shut down the star formation and spread the outer disc, creating the Type III profiles observed in S0 galaxies. The role of mechanisms associated with the spiral-S0 transformation, such as galaxy harassment and ram-pressure stripping, have not been examined yet from the perspective of Type III disc profile formation. In spiral galaxies the environmentally driven spreading of the outer disc could also form the Type III profiles. In addition, in the T & 0 galaxies the outer disc scalelength is slightly larger in the shortest wavelength u-band, compared to infrared. This implies that

82 star formation is present in the outer disc. Whether the increase in outer disc scalelength in the u-band suggests a connection with the extended UV-discs remains a topic of possible further study.

83 84 Bibliography

Aguerri, J. A. L., Balcells, M., and Peletier, R. F. (2001). Growth of galactic bulges by mergers. I. Dense satellites. Astronomy & Astrophysics, 367:428– 442. Allen, P. D., Driver, S. P., Graham, A. W., Cameron, E., Liske, J., and de Propris, R. (2006). The Millennium Galaxy Catalogue: bulge-disc decomposition of 10095 nearby galaxies. Monthly Notices of the Royal Astronom- ical Society, 371:2–18. Athanassoula, E. (2002). Bar-Halo Interaction and Bar Growth. Astrophys- ical Journal Letters, 569:L83–L86. Athanassoula, E., Laurikainen, E., Salo, H., and Bosma, A. (2015). On the nature of the barlens component in barred galaxies: what do boxy/peanut bulges look like when viewed face-on? Monthly Notices of the Royal As- tronomical Society, 454:3843–3863. Athanassoula, E. and Misiriotis, A. (2002). Morphology, photometry and kinematics of N -body bars - I. Three models with diﬀerent halo central concentrations. Monthly Notices of the Royal Astronomical Society, 330:35–52. Athanassoula, E., Morin, S., Wozniak, H., Puy, D., Pierce, M. J., Lombard, J., and Bosma, A. (1990). The shape of bars in early-type barred galaxies. Monthly Notices of the Royal Astronomical Society, 245:130–139. Athanassoula, E., Rodionov, S. A., Peschken, N., and Lambert, J. C. (2016). Forming Disk Galaxies in Wet Major Mergers. I. Three Fiducial Examples. Astrophysical Journal, 821:90. Aumer, M., White, S. D. M., Naab, T., and Scannapieco, C. (2013). Towards a more realistic population of bright spiral galaxies in cosmological simulations. Monthly Notices of the Royal Astronomical Society, 434:3142–3164. Azzollini, R., Trujillo, I., and Beckman, J. E. (2008a). Color Proﬁles of Disk Galaxies since z ∼ 1: Probing Outer Disk Formation Scenarios. Astro- physical Journal Letters, 679:L69.

85 Azzollini, R., Trujillo, I., and Beckman, J. E. (2008b). Cosmic Evolution of Stellar Disk Truncations: From z ∼ 1 to the Local Universe. Astrophysical Journal, 684:1026–1047.

Bakos, J. and Trujillo, I. (2012). Deep Surface Brightness Proﬁles of Spiral Galaxies from SDSS Stripe82: Touching Stellar Halos. ArXiv e-prints, 1204.3082.

Bakos, J., Trujillo, I., and Pohlen, M. (2008). Color Proﬁles of Spiral Galax- ies: Clues on Outer-Disk Formation Scenarios. Astrophysical Journal Let- ters, 683:L103.

Balogh, M. L., Navarro, J. F., and Morris, S. L. (2000). The Origin of Star Formation Gradients in Rich Galaxy Clusters. Astrophysical Journal, 540:113–121.

Barnes, J. E. (1992). Transformations of galaxies. I - Mergers of equal-mass stellar disks. Astrophysical Journal, 393:484–507.

Barteldrees, A. and Dettmar, R.-J. (1994). Parameters of stellar disks from CCD surface photometry of edge-on galaxies. Astronomy & Astrophysics Supplement Series, 103.

Bertin, E. and Arnouts, S. (1996). SExtractor: Software for source extrac- tion. Astronomy & Astrophysics Supplement Series, 117:393–404.

Binney, J. and Tremaine, S. (2008). Galactic Dynamics: Second Edition. Princeton University Press.

Bland-Hawthorn, J., Vlaji´c,M., Freeman, K. C., and Draine, B. T. (2005). NGC 300: An Extremely Faint, Outer Stellar Disk Observed to 10 Scale Lengths. Astrophysical Journal, 629:239–249.

Borlaff, A., Eliche-Moral, M. C., Rodr´ıguez-Pérez,C., Querejeta, M., Tapia, T., Pérez-González,P. G., Zamorano, J., Gallego, J., and Beckman, J. (2014). Formation of S0 galaxies through mergers. Antitruncated stellar discs resulting from major mergers . Astronomy & Astrophysics, 570:A103.

Bosma, A. (1978). The distribution and kinematics of neutral hydrogen in spiral galaxies of various morphological types. PhD thesis, PhD Thesis, Groningen Univ., (1978).

Bournaud, F. (2016). Bulge growth through disc instabilities in high-redshift galaxies. In Laurikainen, E., Peletier, R., and Gadotti, D., editors, Galactic Bulges, pages 355–390. Springer International Publishing.

86 Bournaud, F., Elmegreen, B. G., and Elmegreen, D. M. (2007). Rapid Forma- tion of Exponential Disks and Bulges at High Redshift from the Dynamical Evolution of Clump-Cluster and Chain Galaxies. Astrophysical Journal, 670:237–248.

Boylan-Kolchin, M., Ma, C.-P., and Quataert, E. (2008). Dynamical friction and galaxy merging time-scales. Monthly Notices of the Royal Astronomical Society, 383:93–101.

Boylan-Kolchin, M., Springel, V., White, S. D. M., Jenkins, A., and Lemson, G. (2009). Resolving cosmic structure formation with the Millennium-II Simulation. Monthly Notices of the Royal Astronomical Society, 398:1150– 1164.

Brook, C. B., Governato, F., Roˇskar, R., Stinson, G., Brooks, A. M., Wad- sley, J., Quinn, T., Gibson, B. K., Snaith, O., Pilkington, K., House, E., and Pontzen, A. (2011). Hierarchical formation of bulgeless galaxies: why outﬂows have low angular momentum. Monthly Notices of the Royal As- tronomical Society, 415:1051–1060.

Brook, C. B., Stinson, G., Gibson, B. K., Roˇskar, R., Wadsley, J., and Quinn, T. (2012). Hierarchical formation of bulgeless galaxies - II. Redistribution of angular momentum via galactic fountains. Monthly Notices of the Royal Astronomical Society, 419:771–779.

Brooks, A. and Christensen, C. (2016). Bulge formation via mergers in cosmological simulations. In Laurikainen, E., Peletier, R., and Gadotti, D., editors, Galactic Bulges, pages 317–353. Springer International Publishing.

Buta, R. and Combes, F. (1996). Galactic Rings. Fundamentals of Cosmic Physics, 17:95–281.

Buta, R. J., Sheth, K., Athanassoula, E., Bosma, A., Knapen, J. H., Lau- rikainen, E., Salo, H., Elmegreen, D., Ho, L. C., Zaritsky, D., Courtois, H., Hinz, J. L., Muñoz-Mateos,J.-C., Kim, T., Regan, M. W., Gadotti, D. A., Gil de Paz, A., Laine, J., Menéndez-Delmestre,K., Comerón,S., Erroz Ferrer, S., Seibert, M., Mizusawa, T., Holwerda, B., and Madore, B. F. (2015). A Classical Morphological Analysis of Galaxies in the Spitzer Survey of Stellar Structure in Galaxies (S4G). The Astrophysical Journal Supplement Series, 217:32.

Cameron, E., Driver, S. P., Graham, A. W., and Liske, J. (2009). The Millen- nium Galaxy Catalogue: Exploring the Color-Concentration Bimodality via Bulge-Disk Decomposition. Astrophysical Journal, 699:105–117.

87 Cappellari, M., Emsellem, E., Krajnovi´c,D., McDermid, R. M., Serra, P., Alatalo, K., Blitz, L., Bois, M., Bournaud, F., Bureau, M., Davies, R. L., Davis, T. A., de Zeeuw, P. T., Khochfar, S., Kuntschner, H., Lablanche, P.- Y., Morganti, R., Naab, T., Oosterloo, T., Sarzi, M., Scott, N., Weijmans, A.-M., and Young, L. M. (2011). The ATLAS3D project - VII. A new look at the morphology of nearby galaxies: the kinematic morphology-density relation. Monthly Notices of the Royal Astronomical Society, 416:1680– 1696.

Cayatte, V., van Gorkom, J. H., Balkowski, C., and Kotanyi, C. (1990). VLA observations of neutral hydrogen in Virgo Cluster galaxies. I - The Atlas. The Astronomical Journal, 100:604–634.

Chung, A., van Gorkom, J. H., Kenney, J. D. P., Crowl, H., and Vollmer, B. (2009). VLA Imaging of Virgo Spirals in Atomic Gas (VIVA). I. The Atlas and the H I Properties. The Astronomical Journal, 138:1741–1816.

Chung, A., van Gorkom, J. H., Kenney, J. D. P., and Vollmer, B. (2007). Virgo Galaxies with Long One-sided H I Tails. Astrophysical Journal Let- ters, 659:L115–L119.

Comerón,S., Elmegreen, B. G., Salo, H., Laurikainen, E., Athanassoula, E., Bosma, A., Knapen, J. H., Gadotti, D. A., Sheth, K., Hinz, J. L., Regan, M. W., Gil de Paz, A., Muñoz-Mateos,J. C., Menéndez-Delmestre,K., Seibert, M., Kim, T., Mizusawa, T., Laine, J., Ho, L. C., and Holwerda, B. (2012). Breaks in Thin and Thick Disks of Edge-on Galaxies Imaged in the Spitzer Survey Stellar Structure in Galaxies (S4G). Astrophysical Journal, 759:98.

Comerón,S., Salo, H., Laurikainen, E., Knapen, J. H., Buta, R. J., Herrera- Endoqui, M., Laine, J., Holwerda, B. W., Sheth, K., Regan, M. W., Hinz, J. L., Muñoz-Mateos,J. C., Gil de Paz, A., Menéndez-Delmestre, K., Seib- ert, M., Mizusawa, T., Kim, T., Erroz-Ferrer, S., Gadotti, D. A., Athanas- soula, E., Bosma, A., and Ho, L. C. (2014). ARRAKIS: atlas of resonance rings as known in the S4G. Astronomy & Astrophysics, 562:A121.

Conselice, C. J., Rajgor, S., and Myers, R. (2008). The structures of distant galaxies - I. Galaxy structures and the merger rate to z ∼ 3 in the Hubble Ultra-Deep Field. Monthly Notices of the Royal Astronomical Society, 386:909–927.

Cox, T. J., Jonsson, P., Primack, J. R., and Somerville, R. S. (2006). Feed- back in simulations of disc-galaxy major mergers. Monthly Notices of the Royal Astronomical Society, 373:1013–1038.

88 Crain, R. A., Schaye, J., Bower, R. G., Furlong, M., Schaller, M., Theuns, T., Dalla Vecchia, C., Frenk, C. S., McCarthy, I. G., Helly, J. C., Jenkins, A., Rosas-Guevara, Y. M., White, S. D. M., and Trayford, J. W. (2015). The EAGLE simulations of galaxy formation: calibration of subgrid physics and model variations. Monthly Notices of the Royal Astronomical Society, 450:1937–1961.

Crampin, D. J. and Hoyle, F. (1964). On the Angular-Momentum Distribu- tion in the Disks of Spiral Galaxies. Astrophysical Journal, 140:99.

Dahari, O. (1984). Companions of Seyfert galaxies - A statistical survey. The Astronomical Journal, 89:966–974.

Dalcanton, J. J., Spergel, D. N., and Summers, F. J. (1997). The Formation of Disk Galaxies. Astrophysical Journal, 482:659–676. de Blok, W. J. G. (2010). The Core-Cusp Problem. Advances in Astronomy, 2010:5. de Grijs, R., Kregel, M., and Wesson, K. H. (2001). Radially truncated galactic discs. Monthly Notices of the Royal Astronomical Society, 324:1074– 1086. de Jong, R. S. (1996). Near-infrared and optical broadband surface photometry of 86 face-on disk dominated galaxies. IV. Using color profiles to study stellar and dust content of galaxies. Astronomy & Astrophysics, 313:377–395. de Souza, R. E., Gadotti, D. A., and dos Anjos, S. (2004). BUDDA: A New Two-dimensional Bulge/Disk Decomposition Code for Detailed Struc- tural Analysis of Galaxies. The Astrophysical Journal Supplement Series, 153:411–427. de Vaucouleurs, G. (1948). Recherches sur les Nebuleuses Extragalactiques. Annales d’Astrophysique, 11:247–287. de Vaucouleurs, G. (1959a). Classification and Morphology of External Galaxies. Handbuch der Physik, 53:275. de Vaucouleurs, G. (1959b). Photoelectric Photometry of Messier 33 IN the u, b, v, System. Astrophysical Journal, 130:728. de Vaucouleurs, G., de Vaucouleurs, A., Corwin, Jr., H. G., Buta, R. J., Pa- turel, G., and Fouqué,P. (1991). Third Reference Catalogue of Bright Galaxies. Volume I: Explanations and references. Volume II: Data for galaxies between 0h and 12h. Volume III: Data for galaxies between 12h and 24h.

89 Debattista, V. P., Mayer, L., Carollo, C. M., Moore, B., Wadsley, J., and Quinn, T. (2006). The Secular Evolution of Disk Structural Parameters. Astrophysical Journal, 645:209–227.

DeBuhr, J., Quataert, E., Ma, C.-P., and Hopkins, P. (2010). Self-regulated black hole growth via momentum deposition in galaxy merger simulations. Monthly Notices of the Royal Astronomical Society, 406:L55–L59.

D´ıaz-Garc´ıa,S., Salo, H., Laurikainen, E., and Herrera-Endoqui, M. (2016). Characterization of galactic bars from 3.6 µm S4G imaging. Astronomy & Astrophysics, 587:A160.

Doroshkevich, A. G. (1970). Spatial structure of perturbations and origin of galactic rotation in ﬂuctuation theory. Astrophysics, 6:320–330.

Draine, B. T. and Lee, H. M. (1984). Optical properties of interstellar graphite and silicate grains. Astrophysical Journal, 285:89–108.

Dressler, A. (1980). Galaxy morphology in rich clusters - Implications for the formation and evolution of galaxies. Astrophysical Journal, 236:351–365.

Driver, S. P., Allen, P. D., Graham, A. W., Cameron, E., Liske, J., Ellis, S. C., Cross, N. J. G., De Propris, R., Phillipps, S., and Couch, W. J. (2006). The Millennium Galaxy Catalogue: morphological classiﬁcation and bimodality in the colour-concentration plane. Monthly Notices of the Royal Astronomical Society, 368:414–434.

Duc, P.-A., Cuillandre, J.-C., Karabal, E., Cappellari, M., Alatalo, K., Blitz, L., Bournaud, F., Bureau, M., Crocker, A. F., Davies, R. L., Davis, T. A., de Zeeuw, P. T., Emsellem, E., Khochfar, S., Krajnovi´c,D., Kuntschner, H., McDermid, R. M., Michel-Dansac, L., Morganti, R., Naab, T., Oost- erloo, T., Paudel, S., Sarzi, M., Scott, N., Serra, P., Weijmans, A.-M., and Young, L. M. (2015). The ATLAS3D project - XXIX. The new look of early- type galaxies and surrounding ﬁelds disclosed by extremely deep optical images. Monthly Notices of the Royal Astronomical Society, 446:120–143.

Dutton, A. A. (2009). On the origin of exponential galaxy discs. Monthly Notices of the Royal Astronomical Society, 396:121–140.

Eliche-Moral, M. C., Borlaﬀ, A., Beckman, J. E., and Guti´errez,L. (2015). Photometric scaling relations of anti-truncated stellar discs in S0-Scd galaxies. Astronomy & Astrophysics, 580:A33.

Elmegreen, B. G. and Hunter, D. A. (2006). Radial Proﬁles of Star Formation in the Far Outer Regions of Galaxy Disks. Astrophysical Journal, 636:712– 720.

90 Elmegreen, B. G. and Struck, C. (2013). Exponential Galaxy Disks from Stellar Scattering. Astrophysical Journal Letters, 775:L35.

Elmegreen, D. M. and Elmegreen, B. G. (1984). Blue and near-infrared surface photometry of spiral structure in 34 nonbarred grand design and ﬂocculent galaxies. The Astrophysical Journal Supplement Series, 54:127– 149.

Elmegreen, D. M. and Elmegreen, B. G. (1987). Arm classiﬁcations for spiral galaxies. Astrophysical Journal, 314:3–9.

Emsellem, E., Cappellari, M., Krajnovi´c,D., Alatalo, K., Blitz, L., Bois, M., Bournaud, F., Bureau, M., Davies, R. L., Davis, T. A., de Zeeuw, P. T., Khochfar, S., Kuntschner, H., Lablanche, P.-Y., McDermid, R. M., Morganti, R., Naab, T., Oosterloo, T., Sarzi, M., Scott, N., Serra, P., van de Ven, G., Weijmans, A.-M., and Young, L. M. (2011). The ATLAS3D project - III. A census of the stellar angular momentum within the eﬀective radius of early-type galaxies: unveiling the distribution of fast and slow rotators. Monthly Notices of the Royal Astronomical Society, 414:888–912.

Erroz-Ferrer, S., Knapen, J. H., Leaman, R., D´ıaz-Garc´ıa, S., Salo, H., Laurikainen, E., Querejeta, M., Mu˜noz-Mateos,J. C., Athanassoula, E., Bosma, A., Comer´on,S., Elmegreen, B. G., and Mart´ınez-Valpuesta, I. (2016). Hα kinematics of S4G spiral galaxies - III. Inner rotation curves. Monthly Notices of the Royal Astronomical Society, 458:1199–1213.

Erwin, P. (2015). IMFIT: A Fast, Flexible New Program for Astronomical Image Fitting. Astrophysical Journal, 799:226.

Erwin, P., Beckman, J. E., and Pohlen, M. (2005). Antitruncation of Disks in Early-Type Barred Galaxies. Astrophysical Journal Letters, 626:L81–L84.

Erwin, P., Guti´errez,L., and Beckman, J. E. (2012). A Strong Dichotomy in S0 Disk Proﬁles between the Virgo Cluster and the Field. Astrophysical Journal Letters, 744:L11.

Erwin, P., Pohlen, M., and Beckman, J. E. (2008a). The Outer Disks of Early-Type Galaxies. I. Surface-Brightness Proﬁles of Barred Galaxies. The Astronomical Journal, 135:20–54.

Erwin, P., Pohlen, M., Beckman, J. E., Guti´errez,L., and Aladro, R. (2008b). The Outer Structure of Galactic Disks: Connections Between Bars, Disks, and Environments. In Knapen, J. H., Mahoney, T. J., and Vazdekis, A., editors, Pathways Through an Eclectic Universe, volume 390 of Astronom- ical Society of the Paciﬁc Conference Series, page 251.

91 Eskew, M., Zaritsky, D., and Meidt, S. (2012). Converting from 3.6 and 4.5 µm Fluxes to Stellar Mass. The Astronomical Journal, 143:139.

Eskridge, P. B., Frogel, J. A., Pogge, R. W., Quillen, A. C., Davies, R. L., DePoy, D. L., Houdashelt, M. L., Kuchinski, L. E., Ram´ırez,S. V., Sell- gren, K., Terndrup, D. M., and Tiede, G. P. (2000). The Frequency of Barred Spiral Galaxies in the Near-Infrared. The Astronomical Journal, 119:536–544.

Fall, S. M. and Efstathiou, G. (1980). Formation and rotation of disc galaxies with haloes. Monthly Notices of the Royal Astronomical Society, 193:189– 206.

Farouki, R. and Shapiro, S. L. (1981). Computer simulations of environmental inﬂuences on galaxy evolution in dense clusters. II - Rapid tidal encounters. Astrophysical Journal, 243:32–41.

Fathi, K., Allen, M., Boch, T., Hatziminaoglou, E., and Peletier, R. F. (2010). Scalelength of disc galaxies. Monthly Notices of the Royal Astronomical Society, 406:1595–1608.

Fathi, K., Gatchell, M., Hatziminaoglou, E., and Epinat, B. (2012). Disc scalelengths out to redshift 5.8. Monthly Notices of the Royal Astronomical Society, 423:L112–L116.

Fazio, G. G., Hora, J. L., Allen, L. E., Ashby, M. L. N., Barmby, P., Deutsch, L. K., Huang, J.-S., Kleiner, S., Marengo, M., Megeath, S. T., Melnick, G. J., Pahre, M. A., Patten, B. M., Polizotti, J., Smith, H. A., Taylor, R. S., Wang, Z., Willner, S. P., Hoffmann, W. F., Pipher, J. L., Forrest, W. J., McMurty, C. W., McCreight, C. R., McKelvey, M. E., McMurray, R. E., Koch, D. G., Moseley, S. H., Arendt, R. G., Mentzell, J. E., Marx, C. T., Losch, P., Mayman, P., Eichhorn, W., Krebs, D., Jhabvala, M., Gezari, D. Y., Fixsen, D. J., Flores, J., Shakoorzadeh, K., Jungo, R., Hakun, C., Workman, L., Karpati, G., Kichak, R., Whitley, R., Mann, S., Tollestrup, E. V., Eisenhardt, P., Stern, D., Gorjian, V., Bhattacharya, B., Carey, S., Nelson, B. O., Glaccum, W. J., Lacy, M., Lowrance, P. J., Laine, S., Reach, W. T., Stauffer, J. A., Surace, J. A., Wilson, G., Wright, E. L., Hoffman, A., Domingo, G., and Cohen, M. (2004). The Infrared Array Camera (IRAC) for the Spitzer Space Telescope. The Astrophysical Journal Supplement Series, 154:10–17.

Fisher, D. B. and Drory, N. (2016). An observational guide to identify- ing pseudobulges and classical bulges in disc galaxies. In Laurikainen, E., Peletier, R., and Gadotti, D., editors, Galactic Bulges, pages 41–75. Springer International Publishing.

92 Florido, E., Battaner, E., Guijarro, A., Garz´on,F., and Jim´enez-Vicente, J. (2001). Near infrared observations of the truncation of stellar disks. Astronomy & Astrophysics, 378:82–96.

Foyle, K., Courteau, S., and Thacker, R. J. (2008). An N-body/SPH study of isolated galaxy mass density proﬁles. Monthly Notices of the Royal Astronomical Society, 386:1821–1844.

Freeman, K. C. (1970). On the Disks of Spiral and so Galaxies. Astrophysical Journal, 160:811.

Gadotti, D. A. (2008). Image decomposition of barred galaxies and AGN hosts. Monthly Notices of the Royal Astronomical Society, 384:420–439.

Genzel, R., Tacconi, L. J., Eisenhauer, F., F¨orsterSchreiber, N. M., Cimatti, A., Daddi, E., Bouch´e,N., Davies, R., Lehnert, M. D., Lutz, D., Nes- vadba, N., Verma, A., Abuter, R., Shapiro, K., Sternberg, A., Renzini, A., Kong, X., Arimoto, N., and Mignoli, M. (2006). The rapid formation of a large rotating disk galaxy three billion years after the Big Bang. Nature, 442:786–789.

Gerin, M., Combes, F., and Athanassoula, E. (1990). The inﬂuence of galaxy interactions on stellar bars. Astronomy & Astrophysics, 230:37–54.

Gil de Paz, A., Madore, B. F., Boissier, S., Swaters, R., Popescu, C. C., Tuﬀs, R. J., Sheth, K., Kennicutt, Jr., R. C., Bianchi, L., Thilker, D., and Martin, D. C. (2005). Discovery of an Extended Ultraviolet Disk in the Nearby Galaxy NGC 4625. Astrophysical Journal Letters, 627:L29–L32.

Governato, F., Brook, C., Mayer, L., Brooks, A., Rhee, G., Wadsley, J., Jonsson, P., Willman, B., Stinson, G., Quinn, T., and Madau, P. (2010). Bulgeless dwarf galaxies and dark matter cores from supernova-driven out- ﬂows. Nature, 463:203–206.

Gunn, J. E. and Gott, III, J. R. (1972). On the Infall of Matter Into Clusters of Galaxies and Some Eﬀects on Their Evolution. Astrophysical Journal, 176:1.

Guth, A. H. (1981). Inﬂationary universe: A possible solution to the horizon and ﬂatness problems. Physical Review D (Particles and Fields), 23:347– 356.

Guti´errez, L., Erwin, P., Aladro, R., and Beckman, J. E. (2011). The Outer Disks of Early-type Galaxies. II. Surface-brightness Proﬁles of Un- barred Galaxies and Trends with Hubble Type. The Astronomical Journal, 142:145.

93 Häußler,B., Bamford, S. P., Vika, M., Rojas, A. L., Barden, M., Kelvin, L. S., Alpaslan, M., Robotham, A. S. G., Driver, S. P., Baldry, I. K., Brough, S., Hopkins, A. M., Liske, J., Nichol, R. C., Popescu, C. C., and Tuffs, R. J. (2013). MegaMorph - multiwavelength measurement of galaxy structure: complete Sérsicprofile information from modern surveys. Monthly Notices of the Royal Astronomical Society, 430:330–369.

Haywood, M. (2008). Radial mixing and the transition between the thick and thin Galactic discs. Monthly Notices of the Royal Astronomical Society, 388:1175–1184.

Head, J. T. C. G., Lucey, J. R., and Hudson, M. J. (2015). Beyond S´ersic + exponential disc morphologies in the Coma Cluster. Monthly Notices of the Royal Astronomical Society, 453:3729–3753.

Hernquist, L. and Spergel, D. N. (1992). Formation of shells in major mergers. Astrophysical Journal Letters, 399:L117–L120.

Herpich, J., Stinson, G. S., Dutton, A. A., Rix, H.-W., Martig, M., Roˇskar, R., Macci`o,A. V., Quinn, T. R., and Wadsley, J. (2015). How to bend galaxy disc proﬁles: the role of halo spin. Monthly Notices of the Royal Astronomical Society, 448:L99–L103.

Herrera-Endoqui, M., D´ıaz-Garc´ıa,S., Laurikainen, E., and Salo, H. (2015). Catalogue of the morphological features in the Spitzer Survey of Stellar Structure in Galaxies (S4G). Astronomy & Astrophysics, 582:A86.

Herrmann, K. A., Hunter, D. A., and Elmegreen, B. G. (2013). Surface Brightness Proﬁles of Dwarf Galaxies. I. Proﬁles and Statistics. The As- tronomical Journal, 146:104.

Hohl, F. (1971). Numerical Experiments with a Disk of Stars. Astrophysical Journal, 168:343.

Hubble, E. and Humason, M. L. (1931). The Velocity-Distance Relation among Extra-Galactic Nebulae. Astrophysical Journal, 74:43.

Hubble, E. P. (1936). Realm of the Nebulae. Yale University Press.

Huchra, J. P., Macri, L. M., Masters, K. L., Jarrett, T. H., Berlind, P., Calkins, M., Crook, A. C., Cutri, R., Erdoˇgdu,P., Falco, E., George, T., Hutcheson, C. M., Lahav, O., Mader, J., Mink, J. D., Martimbeau, N., Schneider, S., Skrutskie, M., Tokarz, S., and Westover, M. (2012). The 2MASS Redshift Survey — Description and Data Release. The Astrophys- ical Journal Supplement Series, 199:26.

94 Hunter, D. A. and Elmegreen, B. G. (2006). Broadband Imaging of a Large Sample of Irregular Galaxies. The Astrophysical Journal Supplement Se- ries, 162:49–79.

Hunter, D. A., Elmegreen, B. G., Oh, S.-H., Anderson, E., Nordgren, T. E., Massey, P., Wilsey, N., and Riabokin, M. (2011). The Outer Disks of Dwarf Irregular Galaxies. The Astronomical Journal, 142:121.

Hunter, D. A. and Gallagher, III, J. S. (1985). Star-forming properties and histories of dwarf irregular galaxies - Down but not out. The Astrophysical Journal Supplement Series, 58:533–560.

Ida, S., Kokubo, E., and Makino, J. (1993). The Origin of Anisotropic Velocity Dispersion of Particles in a Disc Potential. Monthly Notices of the Royal Astronomical Society, 263:875.

Janz, J., Laurikainen, E., Lisker, T., Salo, H., Peletier, R. F., Niemi, S.- M., Toloba, E., Hensler, G., Falc´on-Barroso,J., Boselli, A., den Brok, M., Hansson, K. S. A., Meyer, H. T., Ry´s,A., and Paudel, S. (2014). A Near- infrared Census of the Multicomponent Stellar Structure of Early-type Dwarf Galaxies in the Virgo Cluster. Astrophysical Journal, 786:105.

Jarrett, T. H., Chester, T., Cutri, R., Schneider, S., Skrutskie, M., and Huchra, J. P. (2000). 2MASS Extended Source Catalog: Overview and Algorithms. The Astronomical Journal, 119:2498–2531.

Jedrzejewski, R. I. (1987). CCD surface photometry of elliptical galaxies. I - Observations, reduction and results. Monthly Notices of the Royal Astronomical Society, 226:747–768.

Kauﬀmann, G., White, S. D. M., Heckman, T. M., M´enard,B., Brinchmann, J., Charlot, S., Tremonti, C., and Brinkmann, J. (2004). The environmental dependence of the relations between stellar mass, structure, star formation and nuclear activity in galaxies. Monthly Notices of the Royal Astronomical Society, 353:713–731.

Kenney, J. D. P., van Gorkom, J. H., and Vollmer, B. (2004). VLA H I Observations of Gas Stripping in the Virgo Cluster Spiral NGC 4522. The Astronomical Journal, 127:3361–3374.

Kennicutt, Jr., R. C. (1989). The star formation law in galactic disks. As- trophysical Journal, 344:685–703.

Kennicutt, Jr., R. C. (1998). The Global Schmidt Law in Star-forming Galax- ies. Astrophysical Journal, 498:541–552.

95 Kennicutt, Jr., R. C., Armus, L., Bendo, G., Calzetti, D., Dale, D. A., Draine, B. T., Engelbracht, C. W., Gordon, K. D., Grauer, A. D., Helou, G., Hollenbach, D. J., Jarrett, T. H., Kewley, L. J., Leitherer, C., Li, A., Malhotra, S., Regan, M. W., Rieke, G. H., Rieke, M. J., Roussel, H., Smith, J.-D. T., Thornley, M. D., and Walter, F. (2003). SINGS: The SIRTF Nearby Galaxies Survey. Publications of the Astronomical Society of Paciﬁc, 115:928–952.

Kim, T., Gadotti, D. A., Sheth, K., Athanassoula, E., Bosma, A., Lee, M. G., Madore, B. F., Elmegreen, B., Knapen, J. H., Zaritsky, D., Ho, L. C., Comerón,S., Holwerda, B., Hinz, J. L., Muñoz-Mateos,J.-C., Cis- ternas, M., Erroz-Ferrer, S., Buta, R., Laurikainen, E., Salo, H., Laine, J., Menéndez-Delmestre,K., Regan, M. W., de Swardt, B., Gil de Paz, A., Seibert, M., and Mizusawa, T. (2014). Unveiling the Structure of Barred Galaxies at 3.6 µm with the Spitzer Survey of Stellar Structure in Galaxies (S4G). I. Disk Breaks. Astrophysical Journal, 782:64.

Klypin, A., Kravtsov, A. V., Valenzuela, O., and Prada, F. (1999). Where Are the Missing Galactic Satellites? Astrophysical Journal, 522:82–92.

Knapen, J. H., Erroz-Ferrer, S., Roa, J., Bakos, J., Cisternas, M., Leaman, R., and Szymanek, N. (2014). Optical imaging for the Spitzer Survey of Stellar Structure in Galaxies. Data release and notes on interacting galaxies. Astronomy & Astrophysics, 569:A91.

Kormendy, J. (2016). Elliptical galaxies and bulges of disc galaxies: Sum- mary of progress and outstanding issues. In Laurikainen, E., Peletier, R., and Gadotti, D., editors, Galactic Bulges, pages 431–477. Springer Inter- national Publishing.

Kormendy, J. and Bender, R. (2012). A Revised Parallel-sequence Mor- phological Classiﬁcation of Galaxies: Structure and Formation of S0 and Spheroidal Galaxies. The Astrophysical Journal Supplement Series, 198:2.

Kormendy, J., Drory, N., Bender, R., and Cornell, M. E. (2010). Bulgeless Giant Galaxies Challenge Our Picture of Galaxy Formation by Hierarchical Clustering. Astrophysical Journal, 723:54–80.

Kormendy, J. and Norman, C. A. (1979). Observational constraints on driving mechanisms for spiral density waves. Astrophysical Journal, 233:539– 552.

Kregel, M. and van der Kruit, P. C. (2004). Radial truncations in stellar discs in galaxies. Monthly Notices of the Royal Astronomical Society, 355:143– 146.

96 Kregel, M., van der Kruit, P. C., and de Grijs, R. (2002). Flattening and truncation of stellar discs in edge-on spiral galaxies. Monthly Notices of the Royal Astronomical Society, 334:646–668.

Kuzio de Naray, R. and Spekkens, K. (2011). Do Baryons Alter the Halos of Low Surface Brightness Galaxies? Astrophysical Journal Letters, 741:L29.

Lackner, C. N. and Gunn, J. E. (2012). Astrophysically motivated bulge-disc decompositions of Sloan Digital Sky Survey galaxies. Monthly Notices of the Royal Astronomical Society, 421:2277–2302.

Larson, R. B., Tinsley, B. M., and Caldwell, C. N. (1980). The evolution of disk galaxies and the origin of S0 galaxies. Astrophysical Journal, 237:692– 707.

Laurikainen, E. and Salo, H. (2000). BVRI imaging of M 51-type pairs. II. Bulge and disk parameters. Astronomy & Astrophysics Supplement Series, 141:103–111.

Laurikainen, E. and Salo, H. (2001). BVRI imaging of M51-type interacting galaxy pairs - III. Analysis of the photometric parameters. Monthly Notices of the Royal Astronomical Society, 324:685–698.

Laurikainen, E., Salo, H., Athanassoula, E., Bosma, A., Buta, R., and Janz, J. (2013). Statistics of the structure components in S0s: implications for bar-induced secular evolution. Monthly Notices of the Royal Astronomical Society, 430:3489–3509.

Laurikainen, E., Salo, H., Athanassoula, E., Bosma, A., and Herrera- Endoqui, M. (2014). Milky Way mass galaxies with X-shaped bulges are not rare in the local Universe. Monthly Notices of the Royal Astronomical Society, 444:L80–L84.

Laurikainen, E., Salo, H., and Buta, R. (2004). Comparison of Bar Strengths and Fractions of Bars in Active and Nonactive Galaxies. Astrophysical Journal, 607:103–124.

Laurikainen, E., Salo, H., and Buta, R. (2005). Multicomponent decompositions for a sample of S0 galaxies. Monthly Notices of the Royal Astronom- ical Society, 362:1319–1347.

Laurikainen, E., Salo, H., Buta, R., Knapen, J., Speltincx, T., and Block, D. (2006). Morphology of 15 Southern Early-Type Disk Galaxies. The Astronomical Journal, 132:2634–2652.

97 Laurikainen, E., Salo, H., Buta, R., and Knapen, J. H. (2011). Near-infrared atlas of S0-Sa galaxies (NIRS0S). Monthly Notices of the Royal Astronom- ical Society, 418:1452–1490.

Laurikainen, E., Salo, H., Buta, R., Knapen, J. H., and Comer´on,S. (2010). Photometric scaling relations of lenticular and spiral galaxies. Monthly Notices of the Royal Astronomical Society, 405:1089–1118.

Lelli, F., Fraternali, F., and Verheijen, M. (2014). Evolution of dwarf galaxies: a dynamical perspective. Astronomy & Astrophysics, 563:A27.

Lemonias, J. J., Schiminovich, D., Thilker, D., Wyder, T. K., Martin, D. C., Seibert, M., Treyer, M. A., Bianchi, L., Heckman, T. M., Madore, B. F., and Rich, R. M. (2011). The Space Density of Extended Ultraviolet (XUV) Disks in the Local Universe and Implications for Gas Accretion onto Galax- ies. Astrophysical Journal, 733:74.

Lepp, S. and Shull, J. M. (1983). The kinetic theory of H2 dissociation. Astrophysical Journal, 270:578–582.

Lin, C. C. and Shu, F. H. (1964). On the Spiral Structure of Disk Galaxies. Astrophysical Journal, 140:646.

Lin, D. N. C. and Pringle, J. E. (1987). The formation of the exponential disk in spiral galaxies. Astrophysical Journal Letters, 320:L87–L91.

Mac Low, M.-M. and Klessen, R. S. (2004). Control of star formation by supersonic turbulence. Reviews of Modern Physics, 76:125–194.

MacArthur, L. A., Courteau, S., and Holtzman, J. A. (2003). Structure of Disk-dominated Galaxies. I. Bulge/Disk Parameters, Simulations, and Secular Evolution. Astrophysical Journal, 582:689–722.

Makarov, D., Prugniel, P., Terekhova, N., Courtois, H., and Vauglin, I. (2014). HyperLEDA. III. The catalogue of extragalactic distances. As- tronomy & Astrophysics, 570:A13.

Maltby, D. T., Arag´on-Salamanca,A., Gray, M. E., Hoyos, C., Wolf, C., Jogee, S., and B¨ohm,A. (2015). The environmental dependence of the structure of galactic discs in STAGES S0 galaxies: implications for S0 formation. Monthly Notices of the Royal Astronomical Society, 447:1506– 1530.

Maltby, D. T., Gray, M. E., Aragón-Salamanca,A., Wolf, C., Bell, E. F., Jogee, S., Häußler,B., Barazza, F. D., Böhm,A., and Jahnke, K. (2012a). The environmental dependence of the structure of outer galactic discs in

98 STAGES spiral galaxies. Monthly Notices of the Royal Astronomical Soci- ety, 419:669–686.

Maltby, D. T., Hoyos, C., Gray, M. E., Arag´on-Salamanca, A., and Wolf, C. (2012b). Antitruncated stellar light proﬁles in the outer regions of STAGES spiral galaxies: bulge or disc related? Monthly Notices of the Royal Astronomical Society, 420:2475–2479.

Marino, R. A., Gil de Paz, A., Sánchez, S. F., Sánchez-Blázquez,P., Cardiel, N., Castillo-Morales, A., Pascual, S., V´ılchez, J., Kehrig, C., Mollá,M., Mendez-Abreu, J., Catalán-Torrecilla, C., Florido, E., Perez, I., Ruiz-Lara, T., Ellis, S., López-Sánchez, A. R., GonzálezDelgado, R. M., de Lorenzo- Cáceres,A., Garc´ıa-Benito,R., Galbany, L., Zibetti, S., Cortijo, C., Kali- nova, V., Mast, D., Iglesias-Páramo,J., Papaderos, P., Walcher, C. J., and Bland-Hawthorn, J. (2016). Outer-disk reddening and gas-phase metallicities: The CALIFA connection. Astronomy & Astrophysics, 585:A47.

Marinova, I. and Jogee, S. (2007). Characterizing Bars at z ∼ 0 in the Optical and NIR: Implications for the Evolution of Barred Disks with Redshift. Astrophysical Journal, 659:1176–1197.

Markwardt, C. B. (2009). Non-linear Least-squares Fitting in IDL with MPFIT. In Bohlender, D. A., Durand, D., and Dowler, P., editors, Astro- nomical Data Analysis Software and Systems XVIII, volume 411 of Astro- nomical Society of the Paciﬁc Conference Series, page 251.

Mart´ın-Navarro, I., Bakos, J., Trujillo, I., Knapen, J. H., Athanassoula, E., Bosma, A., Comerón,S., Elmegreen, B. G., Erroz-Ferrer, S., Gadotti, D. A., Gil de Paz, A., Hinz, J. L., Ho, L. C., Holwerda, B. W., Kim, T., Laine, J., Laurikainen, E., Menéndez-Delmestre, K., Mizusawa, T., Muñoz-Mateos,J.-C., Regan, M. W., Salo, H., Seibert, M., and Sheth, K. (2012). A unified picture of breaks and truncations in spiral galaxies from SDSS and S4G imaging. Monthly Notices of the Royal Astronomical Society, 427:1102–1134.

Mart´ın-Navarro, I., Trujillo, I., Knapen, J. H., Bakos, J., and Fliri, J. (2014). Stellar haloes outshine disc truncations in low-inclined spirals. Monthly Notices of the Royal Astronomical Society, 441:2809–2814.

Mart´ınez-Serrano,F. J., Serna, A., Dom´enech-Moral, M., and Dom´ınguez- Tenreiro, R. (2009). Disk Galaxies with Broken Luminosity Proﬁles from Cosmological Simulations. Astrophysical Journal Letters, 705:L133–L137.

Meidt, S. E., Schinnerer, E., Knapen, J. H., Bosma, A., Athanassoula, E., Sheth, K., Buta, R. J., Zaritsky, D., Laurikainen, E., Elmegreen, D., Elmegreen, B. G., Gadotti, D. A., Salo, H., Regan, M., Ho, L. C., Madore,

99 B. F., Hinz, J. L., Skibba, R. A., Gil de Paz, A., Muñoz-Mateos,J.- C., Menéndez-Delmestre,K., Seibert, M., Kim, T., Mizusawa, T., Laine, J., and Comerón,S. (2012). Reconstructing the Stellar Mass Distribu- tions of Galaxies Using S4G IRAC 3.6 and 4.5 µm Images. I. Correcting for Contamination by Polycyclic Aromatic Hydrocarbons, Hot Dust, and Intermediate-age Stars. Astrophysical Journal, 744:17.

Men´endez-Delmestre,K., Sheth, K., Schinnerer, E., Jarrett, T. H., and Scov- ille, N. Z. (2007). A Near-Infrared Study of 2MASS Bars in Local Galaxies: An Anchor for High-Redshift Studies. Astrophysical Journal, 657:790–804.

Mestel, L. (1963). On the galactic law of rotation. Monthly Notices of the Royal Astronomical Society, 126:553.

Mihos, J. C. and Hernquist, L. (1996). Gasdynamics and Starbursts in Major Mergers. Astrophysical Journal, 464:641.

Minchev, I. and Famaey, B. (2010). A New Mechanism for Radial Migration in Galactic Disks: Spiral-Bar Resonance Overlap. Astrophysical Journal, 722:112–121.

Minchev, I., Famaey, B., Quillen, A. C., Di Matteo, P., Combes, F., Vlaji´c, M., Erwin, P., and Bland-Hawthorn, J. (2012). Evolution of galactic discs: multiple patterns, radial migration, and disc outskirts. Astronomy & As- trophysics, 548:A126.

Mo, H. J., Mao, S., and White, S. D. M. (1998). The formation of galactic discs. Monthly Notices of the Royal Astronomical Society, 295:319–336.

Moﬀett, A. J., Kannappan, S. J., Baker, A. J., and Laine, S. (2012). Extended Ultraviolet Disks and Ultraviolet-bright Disks in Low-mass E/S0 Galaxies. Astrophysical Journal, 745:34.

Moore, B., Ghigna, S., Governato, F., Lake, G., Quinn, T., Stadel, J., and Tozzi, P. (1999a). Dark Matter Substructure within Galactic Halos. As- trophysical Journal Letters, 524:L19–L22.

Moore, B., Katz, N., Lake, G., Dressler, A., and Oemler, A. (1996). Galaxy harassment and the evolution of clusters of galaxies. Nature, 379:613–616.

Moore, B., Lake, G., and Katz, N. (1998). Morphological Transformation from Galaxy Harassment. Astrophysical Journal, 495:139–151.

Moore, B., Lake, G., Quinn, T., and Stadel, J. (1999b). On the survival and destruction of spiral galaxies in clusters. Monthly Notices of the Royal Astronomical Society, 304:465–474.

100 Moran, S. M., Heckman, T. M., Kauffmann, G., Davé,R., Catinella, B., Brinchmann, J., Wang, J., Schiminovich, D., Saintonge, A., Gracia-Carpio, J., Tacconi, L., Giovanelli, R., Haynes, M., Fabello, S., Hummels, C., Lemonias, J., and Wu, R. (2012). The GALEX Arecibo SDSS Survey. V. The Relation between the H I Content of Galaxies and Metal Enrichment at Their Outskirts. Astrophysical Journal, 745:66. Muñoz-Mateos,J. C., Sheth, K., Gil de Paz, A., Meidt, S., Athanassoula, E., Bosma, A., Comerón,S., Elmegreen, D. M., Elmegreen, B. G., Erroz- Ferrer, S., Gadotti, D. A., Hinz, J. L., Ho, L. C., Holwerda, B., Jarrett, T. H., Kim, T., Knapen, J. H., Laine, J., Laurikainen, E., Madore, B. F., Menendez-Delmestre, K., Mizusawa, T., Regan, M., Salo, H., Schinnerer, E., Seibert, M., Skibba, R., and Zaritsky, D. (2013). The Impact of Bars on Disk Breaks as Probed by S4G Imaging. Astrophysical Journal, 771:59. Naab, T. and Burkert, A. (2003). Statistical Properties of Collisionless Equal- and Unequal-Mass Merger Remnants of Disk Galaxies. Astrophysical Jour- nal, 597:893–906. Naab, T., Jesseit, R., and Burkert, A. (2006). The influence of gas on the structure of merger remnants. Monthly Notices of the Royal Astronomical Society, 372:839–852. Navarro, J. F. and Benz, W. (1991). Dynamics of cooling gas in galactic dark halos. Astrophysical Journal, 380:320–329. Navarro, J. F. and White, S. D. M. (1994). Simulations of dissipative galaxy formation in hierarchically clustering universes-2. Dynamics of the baryonic component in galactic haloes. Monthly Notices of the Royal Astro- nomical Society, 267:401–412. Noguchi, M. (1987). Close encounter between galaxies. II - Tidal deformation of a disc galaxy stabilized by massive halo. Monthly Notices of the Royal Astronomical Society, 228:635–651. Noguchi, M. (1999). Early Evolution of Disk Galaxies: Formation of Bulges in Clumpy Young Galactic Disks. Astrophysical Journal, 514:77–95. Nordström,B., Mayor, M., Andersen, J., Holmberg, J., Pont, F., Jørgensen, B. R., Olsen, E. H., Udry, S., and Mowlavi, N. (2004). The Geneva- Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of ∼14 000 F and G dwarfs. Astronomy & Astro- physics, 418:989–1019. Näslund,M. and Jörsäter,S. (1997). Surface photometry of the edge-on spiral NGC 4565. I. V-band data and the extended optical warp. Astronomy & Astrophysics, 325:915–922.

101 Ostriker, J. P. and Peebles, P. J. E. (1973). A Numerical Study of the Sta- bility of Flattened Galaxies: or, can Cold Galaxies Survive? Astrophysical Journal, 186:467–480. Patterson, F. S. (1940). The Luminosity Gradient of Messier 33. Harvard College Observatory Bulletin, 914:9–10. Peñarrubia,J., McConnachie, A., and Babul, A. (2006). On the Formation of Extended Galactic Disks by Tidally Disrupted Dwarf Galaxies. Astro- physical Journal Letters, 650:L33–L36. Peletier, R. F., Kutdemir, E., van der Wolk, G., Falcón-Barroso,J., Bacon, R., Bureau, M., Cappellari, M., Davies, R. L., de Zeeuw, P. T., Emsellem, E., Krajnović,D., Kuntschner, H., McDermid, R. M., Sarzi, M., Scott, N., Shapiro, K. L., van den Bosch, R. C. E., and van de Ven, G. (2012). The SAURON project - XX. The Spitzer [3.6] - [4.5] colour in early-type galaxies: colours, colour gradients and inverted scaling relations. Monthly Notices of the Royal Astronomical Society, 419:2031–2053. Peng, C. Y., Ho, L. C., Impey, C. D., and Rix, H.-W. (2002). Detailed Structural Decomposition of Galaxy Images. The Astronomical Journal, 124:266–293. Peng, C. Y., Ho, L. C., Impey, C. D., and Rix, H.-W. (2010). Detailed Decomposition of Galaxy Images. II. Beyond Axisymmetric Models. The Astronomical Journal, 139:2097–2129. Pérez,I. (2004). Truncation of stellar disks in galaxies at z ≈ 1. Astronomy & Astrophysics, 427:L17–L20. Perret, V., Renaud, F., Epinat, B., Amram, P., Bournaud, F., Contini, T., Teyssier, R., and Lambert, J.-C. (2014). Evolution of the mass, size, and star formation rate in high redshift merging galaxies. MIRAGE - A new sample of simulations with detailed stellar feedback. Astronomy & Astro- physics, 562:A1. Pohlen, M., Dettmar, R.-J., and Lütticke, R. (2000). Cut-off radii of galactic disks . A new statistical study on the truncation of galactic disks. Astron- omy & Astrophysics, 357:L1–L4. Pohlen, M., Dettmar, R.-J., Lütticke, R., and Aronica, G. (2002). Outer edges of face-on spiral galaxies. Deep optical imaging of NGC 5923, UGC 9837 and NGC 5434. Astronomy & Astrophysics, 392:807–816. Pohlen, M. and Trujillo, I. (2006). The structure of galactic disks. Studying late-type spiral galaxies using SDSS. Astronomy & Astrophysics, 454:759– 772.

102 Pohlen, M., Zaroubi, S., Peletier, R. F., and Dettmar, R.-J. (2007). On the three-dimensional structure of edge-on disc galaxies. Monthly Notices of the Royal Astronomical Society, 378:594–616.

Quinn, P. J. (1984). On the formation and dynamics of shells around elliptical galaxies. Astrophysical Journal, 279:596–609.

Radburn-Smith, D. J., Roˇskar, R., Debattista, V. P., Dalcanton, J. J., Stre- ich, D., de Jong, R. S., Vlaji´c,M., Holwerda, B. W., Purcell, C. W., Dolphin, A. E., and Zucker, D. B. (2012). Outer-disk Populations in NGC 7793: Evidence for Stellar Radial Migration. Astrophysical Jour- nal, 753:138.

Reynolds, J. H. (1913). The light curve of the Andromeda nebula (NGC 224). Monthly Notices of the Royal Astronomical Society, 74:132–136.

Roediger, J. C., Courteau, S., S´anchez-Bl´azquez, P., and McDonald, M. (2012). Stellar Populations and Radial Migrations in Virgo Disk Galaxies. Astrophysical Journal, 758:41.

Roˇskar, R., Debattista, V. P., Quinn, T. R., Stinson, G. S., and Wadsley, J. (2008a). Riding the Spiral Waves: Implications of Stellar Migration for the Properties of Galactic Disks. Astrophysical Journal Letters, 684:L79.

Roˇskar, R., Debattista, V. P., Stinson, G. S., Quinn, T. R., Kaufmann, T., and Wadsley, J. (2008b). Beyond Inside-Out Growth: Formation and Evolution of Disk Outskirts. Astrophysical Journal Letters, 675:L65.

Roˇskar, R., Teyssier, R., Agertz, O., Wetzstein, M., and Moore, B. (2014). A systematic look at the eﬀects of radiative feedback on disc galaxy formation. Monthly Notices of the Royal Astronomical Society, 444:2837–2853.

Rubin, V. C., Ford, Jr., W. K., Thonnard, N., and Burstein, D. (1982). Rotational properties of 23 SB galaxies. Astrophysical Journal, 261:439– 456.

Rubin, V. C., Ford, W. K. J., and . Thonnard, N. (1980). Rotational properties of 21 SC galaxies with a large range of luminosities and radii, from NGC 4605 /R = 4kpc/ to UGC 2885 /R = 122 kpc/. Astrophysical Jour- nal, 238:471–487.

Ruiz-Lara, T., Pérez,I., Florido, E., Sánchez-Blázquez,P., Méndez-Abreu, J., Lyubenova, M., Falcón-Barroso,J., Sánchez-Menguiano, L., Sánchez, S. F., Galbany, L., Garc´ıa-Benito, R., GonzálezDelgado, R. M., Huse- mann, B., Kehrig, C., López-Sánchez, A.´ R., Marino, R. A., Mast, D., Papaderos, P., van de Ven, G., Walcher, C. J., Zibetti, S., and CALIFA

103 Team (2016). No direct coupling between bending of galaxy disc stellar age and light proﬁles. Monthly Notices of the Royal Astronomical Society, 456:L35–L39.

Sachdeva, S. and Saha, K. (2016). Survival of Pure Disk Galaxies over the Last 8 Billion Years. Astrophysical Journal Letters, 820:L4.

Salim, S., Fang, J. J., Rich, R. M., Faber, S. M., and Thilker, D. A. (2012). Galaxy-scale Star Formation on the Red Sequence: The Continued Growth of S0s and the Quiescence of Ellipticals. Astrophysical Journal, 755:105.

Salo, H. (1991). N-body code for simulation of planar disk-disk encounters - Application to tidally triggered bar formation. Astronomy & Astrophysics, 243:118–138.

Salo, H. and Laurikainen, E. (2000a). N-body model for M51 - I. Multiple encounter versus single passage? Monthly Notices of the Royal Astronomical Society, 319:377–392.

Salo, H. and Laurikainen, E. (2000b). N-body model for M51 - II. Inner structure. Monthly Notices of the Royal Astronomical Society, 319:393– 413.

Salo, H., Laurikainen, E., Buta, R., and Knapen, J. H. (2010). Bars do Drive Spiral Density Waves. Astrophysical Journal Letters, 715:L56–L61.

Salo, H., Rautiainen, P., Buta, R., Purcell, G. B., Cobb, M. L., Crocker, D. A., and Laurikainen, E. (1999). The Structure and Dynamics of the Early-Type Resonance Ring Galaxy IC 4214. II. Models. The Astronomical Journal, 117:792–810.

S´anchez-Bl´azquez,P., Courty, S., Gibson, B. K., and Brook, C. B. (2009). The origin of the light distribution in spiral galaxies. Monthly Notices of the Royal Astronomical Society, 398:591–606.

Schaye, J. (2004). Star Formation Thresholds and Galaxy Edges: Why and Where. Astrophysical Journal, 609:667–682.

Sellwood, J. A. (2014). Secular evolution in disk galaxies. Reviews of Modern Physics, 86:1–46.

Sellwood, J. A. and Binney, J. J. (2002). Radial mixing in galactic discs. Monthly Notices of the Royal Astronomical Society, 336:785–796.

Sersic, J. L. (1968). Atlas de galaxias australes. Observatorio Astronomico.

104 Sheth, K., Armus, L., Athanassoula, E., Bosma, A., Gadotti, D., Munoz- Mateos, J. C., Hinz, J., Regan, M., Laurikainen, E., Jarrett, T., Zaritsky, D., Menendez-Delmestre, K., Madore, B., Elmegreen, D., Knapen, J., Salo, H., Schinnerer, E., Kim, T., Ho, L., Elmegreen, B., Buta, R., Cisternas, M., Laine, J., Comeron, S., Donovan Meyer, J., D’Onghia, E., and Salim, S. (2013). Not Dead Yet! Completing Spitzer’s Legacy with Early Type Galaxies. Spitzer Proposal. Sheth, K., Regan, M., Hinz, J. L., Gil de Paz, A., Menéndez-Delmestre, K., Muñoz-Mateos,J.-C., Seibert, M., Kim, T., Laurikainen, E., Salo, H., Gadotti, D. A., Laine, J., Mizusawa, T., Armus, L., Athanassoula, E., Bosma, A., Buta, R. J., Capak, P., Jarrett, T. H., Elmegreen, D. M., Elmegreen, B. G., Knapen, J. H., Koda, J., Helou, G., Ho, L. C., Madore, B. F., Masters, K. L., Mobasher, B., Ogle, P., Peng, C. Y., Schinnerer, E., Surace, J. A., Zaritsky, D., Comerón,S., de Swardt, B., Meidt, S. E., Kasliwal, M., and Aravena, M. (2010). The Spitzer Survey of Stellar Struc- ture in Galaxies (S4G). Publications of the Astronomical Society of Pacific, 122:1397–1414. Silk, J., Di Cintio, A., and Dvorkin, I. (2013). Galaxy formation. ArXiv e-prints, 1312.0107. Smith, J. A., Tucker, D. L., Kent, S., Richmond, M. W., Fukugita, M., Ichikawa, T., Ichikawa, S.-i., Jorgensen, A. M., Uomoto, A., Gunn, J. E., Hamabe, M., Watanabe, M., Tolea, A., Henden, A., Annis, J., Pier, J. R., McKay, T. A., Brinkmann, J., Chen, B., Holtzman, J., Shimasaku, K., and York, D. G. (2002). The u’g’r’i’z’ Standard-Star System. The Astronomical Journal, 123:2121–2144. Spergel, D. N., Bean, R., Doré,O., Nolta, M. R., Bennett, C. L., Dunkley, J., Hinshaw, G., Jarosik, N., Komatsu, E., Page, L., Peiris, H. V., Verde, L., Halpern, M., Hill, R. S., Kogut, A., Limon, M., Meyer, S. S., Odegard, N., Tucker, G. S., Weiland, J. L., Wollack, E., and Wright, E. L. (2007). Three-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observa- tions: Implications for Cosmology. The Astrophysical Journal Supplement Series, 170:377–408. Springel, V., Di Matteo, T., and Hernquist, L. (2005a). Modelling feedback from stars and black holes in galaxy mergers. Monthly Notices of the Royal Astronomical Society, 361:776–794. Springel, V. and Hernquist, L. (2005). Formation of a Spiral Galaxy in a Major Merger. Astrophysical Journal Letters, 622:L9–L12. Springel, V., White, S. D. M., Jenkins, A., Frenk, C. S., Yoshida, N., Gao, L., Navarro, J., Thacker, R., Croton, D., Helly, J., Peacock, J. A., Cole,

105 S., Thomas, P., Couchman, H., Evrard, A., Colberg, J., and Pearce, F. (2005b). Simulations of the formation, evolution and clustering of galaxies and quasars. Nature, 435:629–636.

Steinmetz, M. and Navarro, J. F. (1999). The Cosmological Origin of the Tully-Fisher Relation. Astrophysical Journal, 513:555–560.

Stinson, G. S., Brook, C., Prochaska, J. X., Hennawi, J., Shen, S., Wadsley, J., Pontzen, A., Couchman, H. M. P., Quinn, T., Macci`o,A. V., and Gibson, B. K. (2012). MAGICC haloes: confronting simulations with observations of the circumgalactic medium at z=0. Monthly Notices of the Royal Astronomical Society, 425:1270–1277.

Thilker, D. A., Bianchi, L., Boissier, S., Gil de Paz, A., Madore, B. F., Martin, D. C., Meurer, G. R., Neﬀ, S. G., Rich, R. M., Schiminovich, D., Seibert, M., Wyder, T. K., Barlow, T. A., Byun, Y.-I., Donas, J., Forster, K., Friedman, P. G., Heckman, T. M., Jelinsky, P. N., Lee, Y.-W., Malina, R. F., Milliard, B., Morrissey, P., Siegmund, O. H. W., Small, T., Szalay, A. S., and Welsh, B. Y. (2005). Recent Star Formation in the Extreme Outer Disk of M83. Astrophysical Journal Letters, 619:L79–L82.

Tody, D. (1993). IRAF in the Nineties. In Hanisch, R. J., Brissenden, R. J. V., and Barnes, J., editors, Astronomical Data Analysis Software and Systems II, volume 52 of Astronomical Society of the Paciﬁc Conference Series, page 173.

Toomre, A. (1964). On the gravitational stability of a disk of stars. Astro- physical Journal, 139:1217–1238.

Toomre, A. (1977). Mergers and Some Consequences. In Tinsley, B. M. and Larson, D. Campbell, R. B. G., editors, Evolution of Galaxies and Stellar Populations, page 401.

Toomre, A. and Toomre, J. (1972). Galactic Bridges and Tails. Astrophysical Journal, 178:623–666.

Tremaine, S. D., Ostriker, J. P., and Spitzer, Jr., L. (1975). The formation of the nuclei of galaxies. I - M31. Astrophysical Journal, 196:407–411.

Trujillo, I. and Fliri, J. (2016). Beyond 31 mag arcsec-2: The Frontier of Low Surface Brightness Imaging with the Largest Optical Telescopes. Astro- physical Journal, 823:123.

Trujillo, I. and Pohlen, M. (2005). Stellar Disk Truncations at High z: Prob- ing Inside-Out Galaxy Formation. Astrophysical Journal Letters, 630:L17– L20.

106 Tully, R. B. and Fisher, J. R. (1977). A new method of determining distances to galaxies. Astronomy & Astrophysics, 54:661–673.

Valenzuela, O. and Klypin, A. (2003). Secular bar formation in galaxies with a signiﬁcant amount of dark matter. Monthly Notices of the Royal Astronomical Society, 345:406–422. van den Bergh, S. (1976). A new classiﬁcation system for galaxies. Astro- physical Journal, 206:883–887. van den Bergh, S., Abraham, R. G., Ellis, R. S., Tanvir, N. R., Santiago, B. X., and Glazebrook, K. G. (1996). A Morphological Catalog of Galaxies in the Hubble deep Field. The Astronomical Journal, 112:359. van den Bosch, F. C., Aquino, D., Yang, X., Mo, H. J., Pasquali, A., McIn- tosh, D. H., Weinmann, S. M., and Kang, X. (2008). The importance of satellite quenching for the build-up of the red sequence of present-day galaxies. Monthly Notices of the Royal Astronomical Society, 387:79–91. van der Kruit, P. C. (1979). Optical surface photometry of eight spiral galaxies studied in Westerbork. Astronomy & Astrophysics Supplement Series, 38:15–38. van der Kruit, P. C. (1987). The radial distribution of surface brightness in galactic disks. Astronomy & Astrophysics, 173:59–80. van der Kruit, P. C. and Searle, L. (1981a). Surface photometry of edge-on spiral galaxies. I - A model for the three-dimensional distribution of light in galactic disks. Astronomy & Astrophysics, 95:105–115. van der Kruit, P. C. and Searle, L. (1981b). Surface Photometry of Edge-On Spiral Galaxies. II - the Distribution of Light and Colour in the Disk and Spheroid of NGC891. Astronomy & Astrophysics, 95:116. van der Kruit, P. C. and Searle, L. (1982a). Surface photometry of edge- on spiral galaxies. III - Properties of the three-dimensional distribution of light and mass in disks of spiral galaxies. Astronomy & Astrophysics, 110:61–78. van der Kruit, P. C. and Searle, L. (1982b). Surface photometry of edge-on spiral galaxies. IV - The distribution of light, colour, and mass in the disk and spheroid of NGC 7814. Astronomy & Astrophysics, 110:79–94. van Zee, L. (2001). The Evolutionary Status of Isolated Dwarf Irregular Galaxies. II. Star Formation Histories and Gas Depletion. The Astronom- ical Journal, 121:2003–2019.

107 Velazquez, H. and White, S. D. M. (1999). Sinking satellites and the heating of galaxy discs. Monthly Notices of the Royal Astronomical Society, 304:254–270.

Verley, S., Leon, S., Verdes-Montenegro, L., Combes, F., Sabater, J., Sulen- tic, J., Bergond, G., Espada, D., Garc´ıa,E., Lisenfeld, U., and Odewahn, S. C. (2007). The AMIGA sample of isolated galaxies. V. Quantiﬁcation of the isolation. Astronomy & Astrophysics, 472:121–130.

Vogelsberger, M., Genel, S., Springel, V., Torrey, P., Sijacki, D., Xu, D., Snyder, G., Nelson, D., and Hernquist, L. (2014). Introducing the Illustris Project: simulating the coevolution of dark and visible matter in the Uni- verse. Monthly Notices of the Royal Astronomical Society, 444:1518–1547.

Weinzirl, T., Jogee, S., Khochfar, S., Burkert, A., and Kormendy, J. (2009). Bulge n and B/T in High-Mass Galaxies: Constraints on the Origin of Bulges in Hierarchical Models. Astrophysical Journal, 696:411–447.

Werner, M. W., Roellig, T. L., Low, F. J., Rieke, G. H., Rieke, M., Hoﬀmann, W. F., Young, E., Houck, J. R., Brandl, B., Fazio, G. G., Hora, J. L., Gehrz, R. D., Helou, G., Soifer, B. T., Stauﬀer, J., Keene, J., Eisenhardt, P., Gallagher, D., Gautier, T. N., Irace, W., Lawrence, C. R., Simmons, L., Van Cleve, J. E., Jura, M., Wright, E. L., and Cruikshank, D. P. (2004). The Spitzer Space Telescope Mission. The Astrophysical Journal Supplement Series, 154:1–9.

White, S. D. M. and Rees, M. J. (1978). Core condensation in heavy halos - A two-stage theory for galaxy formation and clustering. Monthly Notices of the Royal Astronomical Society, 183:341–358.

Whyte, L. F., Abraham, R. G., Merriﬁeld, M. R., Eskridge, P. B., Frogel, J. A., and Pogge, R. W. (2002). Morphological classiﬁcation of the OSU Bright Spiral Galaxy Survey. Monthly Notices of the Royal Astronomical Society, 336:1281–1286.

Wielen, R. (1977). The diﬀusion of stellar orbits derived from the observed age-dependence of the velocity dispersion. Astronomy & Astrophysics, 60:263–275.

Yoachim, P., Roˇskar, R., and Debattista, V. P. (2010). Integral Field Unit Spectroscopy of the Stellar Disk Truncation Region of NGC 6155. Astro- physical Journal Letters, 716:L4–L8.

Yoachim, P., Roˇskar, R., and Debattista, V. P. (2012). Spatially Resolved Spectroscopic Star Formation Histories of nearby Disks: Hints of Stellar Migration. Astrophysical Journal, 752:97.

108 Younger, J. D., Cox, T. J., Seth, A. C., and Hernquist, L. (2007). Antitrun- cated Stellar Disks via Minor Mergers. Astrophysical Journal, 670:269–278.

Zaritsky, D. and Christlein, D. (2007). On the Extended Knotted Disks of Galaxies. The Astronomical Journal, 134:135–141.

Zheng, Z., Thilker, D. A., Heckman, T. M., Meurer, G. R., Burgett, W. S., Chambers, K. C., Huber, M. E., Kaiser, N., Magnier, E. A., Metcalfe, N., Price, P. A., Tonry, J. L., Wainscoat, R. J., and Waters, C. (2015). The Structure and Stellar Content of the Outer Disks of Galaxies: A New View from the Pan-STARRS1 Medium Deep Survey. Astrophysical Journal, 800:120.

Zwicky, F. (1933). Die Rotverschiebung von extragalaktischen Nebeln. Hel- vetica Physica Acta, 6:110–127.

Zwicky, F. (1937). On the Masses of Nebulae and of Clusters of Nebulae. Astrophysical Journal, 86:217.

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