Active Galactic Nuclei and Their Neighbours

CCD Photometric Observations of Active Galactic Nuclei and their Neighbours

Traianou Efthalia

A dissertation submitted in partial fulﬁllment of the requirements for the degree of Ptychion (Physics) in Aristotle University of Thessaloniki September 2016

Supervisor:

Manolis Plionis, Professor To my loved ones

Many thanks to: Manolis Plionis for accepting to be my thesis adviser.

ii TABLE OF CONTENTS

DEDICATION ...... ii

LIST OF FIGURES ...... v

LIST OF TABLES ...... ix

LIST OF APPENDICES ...... x

ABSTRACT ...... xi

CHAPTER

I. Introduction ...... 1

II. Active Galactic Nuclei ...... 4

2.1 Early History of AGN’s ...... 4 2.2 AGN Phenomenology ...... 7 2.2.1 Seyfert Galaxies ...... 7 2.2.2 Low Ionization Nuclear Emission-Line Regions(LINERS) 10 2.2.3 ULIRGS ...... 11 2.2.4 Radio Galaxies ...... 12 2.2.5 Quasars or QSO’s ...... 14 2.2.6 Blazars ...... 15 2.3 The Uniﬁcation Paradigm ...... 16 2.4 Beyond the Uniﬁed Model ...... 18

III. Research Goal and Methodology ...... 21

3.1 Torus ...... 21 3.2 Ha Balmer Line ...... 23 3.3 Galaxy-Galaxy Interactions ...... 25 3.4 Our Aim ...... 27

iii IV. Observations ...... 29

4.1 The Telescope ...... 29 4.2 Instrumentation ...... 30 4.3 Preparation and Observations ...... 31 4.3.1 Observations ...... 33

V. Data Analysis ...... 35

5.1 Background ...... 35 5.2 Data Reduction ...... 37 5.2.1 Dark Noise ...... 37 5.2.2 Bias Subtraction ...... 37 5.2.3 Flat Field Correction ...... 38 5.2.4 Fixing the Problems ...... 40 5.2.5 Cosmic Rays Cleaning ...... 40 5.2.6 Galaxy Extinction ...... 41 5.2.7 AB Magnitude ...... 41 5.3 Airmass ...... 44 5.4 Standard Stars ...... 45 5.5 Photometry ...... 48 5.5.1 Aperture Photometry ...... 48 5.5.2 Surface Photometry ...... 50

VI. Results and Conclusions ...... 51

6.1 Comparison with the bibliography ...... 51 6.2 Photometry between AGN and neighbours ...... 53 6.2.1 Individual “Blob” Photometry ...... 66 6.2.2 NGC 7469 ...... 73

APPENDICES ...... 75

BIBLIOGRAPHY ...... 85

iv LIST OF FIGURES

Figure

2.1 Right:A multiwavelength view of the Cygnus A radio-galaxy as captured by Hubble telescope. Image Credit: NASA, ESA, S. Baum and C. O’Dea (RIT), R. Perley and W. Cotton (NRAO/AUI/NSF), and the Hubble Heritage Team (STScI/AURA) . Left:An image montage in optical frequencies of the distant quasar 3C 273. Image Credit: NASA and J. Bahcall (IAS)...... 6 2.2 In this BPT diagram we see the spread of emission-line galaxies from the Sloan Digital Sky Survey (SDSS). This diagram uses 4 strong optical emission lines, [OIII] 5007, [NII] 6583, Ha 6563, and Hb 4861, in order to distinguish galaxies that are dominated by ionization from young stars (green dots, labelled as ”Star-forming Galaxies”), from those that are ionized by an accreting SMBH in their center (Seyfert and LINER galaxies). The solid curve indicates the empirical dividing lines and the dased the theoretical between active galactic nuclei and star-forming (SF) galaxies, based upon the SDSS spectroscopic observations (Kauffmann et al. [46], Kewley and A.Dopita [51]. ... 11 2.3 Stacked 43 GHz image from the radio galaxy Gygnus A, observed with the Global VLBI at 7mm wavelengths. This resolution corresponds to a linear scale of approximate 400 Schwarzschild radii. (Boccardi et al. [11]) ...... 13 2.4 This is the spectra of the first quasar in history. As we can see, the emission lines are shifted because of Doppler Effect(∆λ = u/c, where λ is the wavelength of the line and c the speed of light and u ≪ c). 15 2.5 A unified model of AGNs. The upper right part of the drawing corresponds to high-power sources with the jet emerging from an open torus, the left upper part, represent the low-power sources with the jet emerging from a closed torus. Different morphologies are produced by the orientation of the observer with respect to the obscuring torus. Credit: Beckmann and Shrader [8]...... 18 2.6 Simple schematic of galaxy and AGN evolution (Hickox et al. [39]) . 20

v 3.1 A representation of the 2 phase torus (a homogeneous disk or a clumpy medium) and of the host galaxy structure together with the thermal distribution around the central engine (as presented in the work of Siebenmorgen et al. [80])...... 22 3.2 Electron transitions of the hydrogen atom and the wavelength of the each emission line that stems from...... 24 3.3 Inside the corotation, the gas undergoes negative gravity torques, and looses angular momentum in a rapid central gas inflow towards the central area. In the outer disk, gas would gain angular momentum and fly out to intergalactic space in long tidal tails...... 26 4.1 The Aristarchos telescope of the Helmos Observatory. The telescope has a Ritchey-Cretien optical system with a primary mirror of 2.280m in diameter whereas its focal ratio is f/8 and its focal length reaches the 17.714 meters. Photo credits: Theofanis Matsopoulos...... 30 4.2 An artistic photograph of the observatory during a rare atmospheric phenomenon (sundog and circumnuclear arc) taken by the author. . 33 5.1 An illustration of the collecting procedure of a typical CCD sensor. The photons fill the pixels, which are converted in order to reach the computer as a digital signal...... 36 5.2 Left: A print screen of the adjustments in Aristarchos remote control environment in order to obtain bias. Right: A master bias frame of our observations...... 38 5.3 A snapshot of the CCD cooling procedure using liquid hydrogen in Aristarchos telescope...... 39 5.4 A typical appearance of a master flat. This frame is a combination of 7 frames before our observations and 7 after. We create a master flat for each and every night...... 39 5.5 At left we can see a single ”dirty” image of the NGC 1241 galaxy and its neighbour. At right we can see a combination of three exposures, cleaned from CCD noise and cosmic events...... 41 5.6 A schematic representation of the airmass effect...... 45 5.7 The mAB absolute magnitude vs wavelenght (Å) for the Feige34 spectrophotometric standard star...... 46 5.8 The interface of the Aristarchos Telescope Control Gui environment during an standard star exposure...... 46 5.9 An examble of the aperture photometry. The inner radius measure the star+backround counts, whereas the annulus the sky contribution. 49 6.1 log(L) of the currrent work versus log(L) of Theios et al. [83]. The diagonal line represents the equality line between the different measurements. We can see that there are some differences, but there are several reasons for this. The most crucial are the weather conditions, since we faced problematic photometric atmospheric conditions with heavy cirrus clouds during some of our observation sessions. .... 52

vi 6.2 Left: The Sy2 NGC 3786 and its neighbour NGC 3788. The upper panel shows the SDSS broad-band composite image while the lower panel shows our narrow-band Ha data...... 54 6.3 The pair Sy2 NGC 1320. The upper panel shows the SDSS broad- band composite image while the lower panel shows our narrow-band Ha data...... 55 6.4 The pair Sy2 UGC 12138. The upper panel shows the SDSS broad- band composite image while the lower panel shows our narrow-band Ha data...... 56 6.5 Left: The pair Sy2 NGC 1241. The upper panel shows the SDSS broad-band composite image while the lower panel shows our narrow- band Ha data...... 57 6.6 Left: The pair Sy2 IRAS 00160-0719. The upper panel shows the SDSS broad-band composite image while the lower panel shows our narrow-band Ha data...... 58 6.7 Left: The pair Sy2 MRK 612. The upper panel shows the SDSS broad-band composite image while the lower panel shows our narrow- band Ha data...... 59 6.8 Left: The pair Sy2 NGC 7682. The upper panel shows the SDSS broad-band composite image while the lower panel shows our narrow- band Ha data. These object were selected as control sample. .... 60 6.9 Left: The pair Sy1 NGC 863. The upper panel shows the SDSS broad- band composite image while the lower panel shows our narrow-band Ha data...... 61 6.10 Left: The pair Sy1 NGC 1019. The upper panel shows the SDSS broad-band composite image while the lower panel shows our narrow- band Ha data...... 62 6.11 The pair Sy1 NGC 1194. The upper panel shows the SDSS broad- band composite image while the lower panel shows our narrow-band Ha data...... 63 6.12 Ha F luxbetween versus Ha F luxbackground for the Seyfert 1 case. ... 64 6.13 Ha F luxbetween versus Ha F luxbackground luminosities for the Seyfert 2 cases...... 65 6.14 Ha F luxbetween versus Ha F luxbackground of all our objects. As we can see, the ﬂux diﬀerences between the area that we are interested in, and the background are within the 1σ uncertainty...... 65 6.15 IRAS 00160-0719 and its neighbour objects based on the SIMBAD catalogue...... 66 6.16 The “Blob” suspected of being part of the AGN-neighbour system in circled. The estimated redshift (with KD tree method) placed this blob quite close to Sy 2 galaxy. This is circustantial evidence that IRAS 00160-0719 could be a hidden Sy 1 galaxy...... 67 6.17 The 3D surface plot of this area showing the counts per pixel. ... 67

vii 6.18 This “blob” is estimated to have z = 0.160 0.080. Notice that the estimated by this method AGN redshift deviates significantly from its measured value; however we believe that negligible relative z differences (even if the absolute values are wrong) could indicate a physical association of the “blob” with the AGN-neighbour system. 68 6.19 The selected area around the “blob” as a 3D plot which the x and y axis represent the dimension of our selection and z-axis the count values...... 68 6.20 The second “blob” which we spotted. In this case there is no available estimation of its z...... 69 6.21 “Blob” 2 area in a 3D plot...... 69 6.22 This “blob” is labelled as “blob” 3. Although, there is Ha emission, comparing the estimated “blob” and galaxy redshifts show that it is probably a background object and thus not associated with the AGN-neighbour system...... 70 6.23 “Blob” 3 area in a 3D plot...... 70 6.24 “Blob” 4. Again, based in the SDSS KD-tree method this blob does not appear to belong to the system...... 71 6.25 “Blob 4” area in a 3D plot...... 71 6.26 “Blob” 5. Based in the SDSS KD-tree method this blob does not appear to belong to the system...... 72 6.27 Blob 5 area in a 3D plot...... 72 6.28 NGC 7469. The intense starformation in the arm of NGC 7469 towards the neighbour is significantly higher than that of the opposite side...... 73

A.1 The solution space resulting from the χ2-minimization procedure where both the k constant and zero-point z ﬁtted parameters are free. These results correspond to the last night of observations(09/11/15). Notice that the k and z parameters are quite degenerate, although the accuracy of z is quite satisfying...... 81

viii LIST OF TABLES

Table

2.1 The Grand Unification Scheme based in radio emission ...... 17 3.1 The most used in astronomy lines of Balmer Series...... 23 4.1 Observational sample of galaxy pairs in Ha line. Considered the galaxy redshifts, we used the Continium Red filter in 6680 Åwith an BW 100 Å...... 34 5.1 Differences between mAB magnitudes in sight the atmosphere and through telescope and outside of atmosphere...... 44 5.2 Standard star observations by night...... 47 5.3 Our Measurements in Ha+ Continuum. All fluxes and magnitudes are cleaned from [NII] line and galaxy extinction contamination as we described in ref ...... 50 6.1 The Log(LHa) comparison between this paper and Theios et al. [83] 51

ix LIST OF APPENDICES

Appendix

A. Calculations ...... 76

B. Filter Constant ...... 83

x ABSTRACT

CCD Photometric Observations of Active Galactic Nuclei and their neighbours in order to detect Elements of Physical Interaction

by Traianou Efthalia

The dominant paradigm that explains the diﬀerent variety of Active Galactic

Nuclei (AGN) is the so-called Unification Model of AGN. According to this model, the different AGN phenomenology is the result of the viewing angle with which we observe the AGN nucleus. Although such an explanation is adequate in many cases, there are important indications that other factors, related to the physics of AGN and their triggering mechanisms, as for example galaxy-galaxy interactions and merging, could lead an AGN to be of a specific type or even change its appearance. The primary purpose of this work is to investigate aspects of this later possibility. Briefly, this is a pilot study of Ha narrow band filter observations of of Sy1 and Sy2s which have a close neighbour. The idea which we are attempting to test is whether galaxy close interactions will not only sweap away gas and dust from their host galaxies, but due to angular momentum conservation, large possibly quantities of gas and dust will fall towards the nucleus creating a cocoon and thus possibly masquerading Sy1’s to look like Sy2’s. As a possible imprint of such effective interactions we will attempt to

xi observe star-formation in the regions between the AGN and its neighbour, expecting (if the scenario discussed is valid) that such star-forming intra-galactic regions will be mostly related to Sy2 galaxies with neighbours.

We have observed 4 Sy1 and 7 Sy2 galaxies with the Aristarchos 2.3m telescope, situated at mount Helmos, during 3 observing runs between 15th August and 9th

November 2015. The data were reduced and analysed using the IRAF, Gaia and As- troimage J packages. In order to check the validity of our methodology, we compared results of some of our galaxies with other similar studies and we got a quite satis-

ﬁed consistency. We ﬁnd indications for the existence of an enhanced Ha emission between some AGN-neighbour pairs compared to the average background emission. However, our results are preliminary and in order to derive more conclusive results a much larger sample of AGN must be observed and analysed.

xii ΠΕΡΙΛΗΨΗ

Το βασικό μοντέλο που προσπαθεί να εξηγήσει την ποικιλία των ενεργών γαλαξια- κών πυρήνων (AGN) ονομάζεται ενοποιημένο μοντέλο. Σύμφωνα με αυτό, η διαφορετική

φαινομενολογία των ενεργών γαλαξιακών πυρήνων είναι αποτέλεσμα των διαφορετικών

γωνιών παρατήρησης τους από εμάς. Παρόλο που αυτή η εξήγηση είναι επαρκής σε αρ-

κετές περιπτώσεις, υπάρχουν σημαντικές ενδείξεις ότι άλλοι παράγοντες που σχετίζονται με τη φυσική των ενεργών γαλαξιών και τους μηχανισμούς ενεργοποίησης τους, όπως για

παράδειγμα οι αλληλεπιδράσεις γαλαξιών και η πιθανή συγχώνευση τους θα μπορούσαν

να οδηγήσουν στην αλλαγή του τύπου AGN. Ο κύριος σκοπός αυτής της εργασίας είναι να

ερευνηθούν στοιχεία προς αυτή την κατεύθυνση. Εν συντομία, η εργασία αποτελεί μια πι-

λοτική μελέτη φωτομετρικών παρατηρήσεων, στη γραμμή του υδρογόνου (Ηα), γαλαξιών τύπου Sy1 και Sy2 που έχουν έναν κοντινό συνοδό. Η ιδέα που επιδιώκεται να ελεγχθεί εί-

ναι εάν αλληλεπιδράσεις μεταξύ κοντινών γαλαξιών δεν παρασύρουν μόνο αέριο και σκόνη

από τους συνοδούς, αλλά εάν λόγω της αρχής διατήρησης της στροφορμής μεγάλες ποσό-

τητες αερίου και σκόνης προσπίπτουν προς τον πυρήνα δημιουργώντας ένα κουκούλι με

συνέπεια να "καμουφλάρουν" Sy1 γαλαξίες και να τους αντιλαμβανόμαστε ως Sy2. Πι- θανή ένδειξη τέτοιων δραστικών αλληλεπιδράσεων είναι η ύπαρξη έντονης αστρογένεσης

σε περιοχές ανάμεσα στον ενεργό πυρήνα και τον συνοδό του αναμένοντας ότι (αν αυτό το

μοντέλο είναι σωστό) η αστρογένεση θα είναι πιο έντονη στους Sy2 γαλαξίες και τους γεί-

τονες τους. Από τον Αύγουστο μέχρι τον Οκτώβριου του 2015, παρατηρήσαμε 11 γαλαξίες και των δύο τύπων με το διαμέτρου 2.3 μέτρων τηλεσκόπιο "Αρίσταρχος" που βρίσκεται

στο βουνό Χελμός. Τα δεδομένα αποθορυβοποιήθηκαν και αναλύθηκαν με τα προγράμματα

επεξεργασίας IRAF, ASTROIMAGEJ, GAIA. Με σκοπό να ελέγξουμε τα αποτελέσματα

της φωτομετρικής μεθόδου που ακολουθήσαμε, συγκρίναμε τα αποτελέσματά μας με ένα

κοινό δείγμα γαλαξιών από άλλες εργασίες και τα αποτελέσματα της σύγκρισης ήταν ικανο- ποιητικά. Λεπτομερής ανάλυση των αποτελεσμάτων δείχνουν ότι η εκπομπή στην γραμμή

Ηα είναι πράγματι λίγο μεγαλύτερη στην περιοχή μεταξύ των ζευγών γαλαξιών συγκριτικά

xiii με την εκπομπή του υποβάθρου της εικόνας μας. Ωστόσο, για να καταλήξουμε σε στατιστι- κώς σημαντικά αποτελέσματα για τον έλεγχο της υπόθεσής μας χρειάζονται να γίνουν στο

μέλλον περισσότερες παρατηρήσεις για την απόκτιση μεγαλύτερου στατιστικού δείγματος.

xiv CHAPTER I

Introduction

Galaxies are large collections of gas, dust, dark matter and billions of stars, which usually form a disk in space. The ﬁrst galaxy classiﬁcation was based on their morphological characteristics (Hubble [42]) and categorized galaxies into two main categories;

Spirals and Ellipticals. However, the last few decades our technological achievements have allowed us to study the universe in almost the whole electromagnetic spectrum and such multiwavelength observations have revealed a great variety of galaxy types and of cosmic phenomena.

Going back in time, it was in 1943 when the astronomer Carl Keenan Seyfert observed in the optical band a peculiar object which appeared as an unassuming blue star. The corresponding spectroscopic observations revealed that such peculiar objects showed strong line emission with some lines being Doppler broadened, indicating velocities reaching even ∼8500 km/sec (Seyfert [77]). In addition, radio observations showed that this ”star” was a blaze of light, a luminous powerhouse. Today we know that these objects are galaxies with an accreting supermassive black hole in their center; the so-called Active Galactic Nuclei (AGN). Astronomers have since carried out statistical studies of thousands of active galaxies, finding systematic differences and a variety of activity types, and attempted to relate them to each other. It is still under debate whether the different properties of active galaxies depend on initial

1 conditions or their particular evolutionary history. A very successful model where the different AGN phenomenology is unified under a unique parameter, that of the orientation of the observer’s viewing angle. This model assumes the existence of four fundamental components of the nucleus; the central black hole, the accretion disk, a dust torus surrounding the accretion disk and gas clouds that emit line radiation. Nevertheless, there are still numerous open questions, such as the exact geometry of accretion, the formation and composition of the relativistic jets, the location of the origin and the underlying physical processes of the emission regions in different energy ranges and the AGN triggering mechanism.

Strong indications have been found (Villarroel and Korn [89], Koulouridis et al. [56], Koulouridis et al. [57]) that galaxy mergers and/or tidal interactions between

AGN’s and their neighbours could play an important role in feeding nuclear activity and in determining the AGN phenomenology. During such interactions, the enormous gravitational field of the galaxy bulge affects material in its surrounding. Parts of the gas is stripped of the galaxy being diffused into the intergalactic medium, while due to angular momentum conservation other parts of the gas and dust will spiral in towards the nucleus, virtually providing the fuel for the AGN. The gas stripped from the galaxy during such a violent event can eventually provide the fuel for starformation in regions which are quite dissociated from the galaxy of origin, being traced by strong Balmer line emission.

In this study we intend to investigate this scenario performing narrow band Ha imaging using the 2.3m Aristarchos telescope, the largest telescope in Greece, and properly analysing the data in order to attempt to ﬁnd indications of starformation in the regions between close pairs of galaxies, one of which is either a Sy1 or a Sy2 galaxy. We reduce the CCD imaging data with a variety of indicated astronomical packages, while we also describe in detail the basic steps for the preparation and realization of the observing runs.

2 The structure of this work is as follows. In Chapter II we will present some important deﬁnitions and processes in AGN’s, which are useful for understanding our hypothesis and the disucssion regarding the validity of the uniﬁcation paradigm. In

Chapter III we deﬁne our goals and research methodology. Having established the basic idea of this thesis, we present in Chapter IV information about the Aristar- chos telescope and the instrumentation that we have used, as well as our observing preparations. In Chapter V we introduce the basic background on photometry, as well as one by one the steps of data reduction and photometric calibration. Finally, in Chapter VI we provide the results and our concluding remarks, referring also to future developments of our research project.

3 CHAPTER II

Active Galactic Nuclei

Introduction

In this chapter we will brieﬂy make an introduction to some very basic and important concepts in theories about the AGN appearance and phenomenology. We will see in more detail what deﬁnes a galaxy as active and also what parameters and processes describe it in its most basic and simple level.

2.1 Early History of AGN’s

The ﬁrst sign of the AGN took place in 1908 in a study of the optical spectra of spiral nebulae (Fath [33]), when bright emission lines in the nuclear spectrum of

NGC 1068 had been observed. These lines originate from the atomic transitions of the circumnuclear material of the galaxy NGC1068 is an object that we know today that is a nearby bright active galaxy. But at that time, it was unknown that there are objects outside our galaxy (this is the reason they called the galaxies nebulae) and other fundamental facts such as general relativity. A few years later, Slipher [81] demonstrated these lines to be very broad, suggesting fast motions in the nucleus of NGC 1068. The next remarkable discovery was done by H. Curtis, which he took the

ﬁrst image of an AGN and notably, with the ﬁrst and for many years the only image

4 of a jet. His description of the optical jet is still famous today: ”The image of M87 showed a curious straight ray, apparently connected with the nucleus by a thin line of matter.” (Curtis [19]) . Seyfert ([77]), as we have mentioned before, studied the spectra of spiral nebulae in detail and found that many of them have bright nuclei and broad emission lines. Another signiﬁcant step was taken by Khachikian and D.

[54]in 1971 when they came to the conclusion that there exist two distinct classes of the so-called Seyfert galaxies, based on their spectra and appearance, those of Type

1 and Type 2. Astonishing new insights of the AGN were obtained, when it became

possible to observe the sky in other frequencies, diﬀerent from the optical regime. A

new era of AGN studies started with the detection of bright galaxies and quasars in the radiowaves in the middle of the 20th century. After the discovery of radio emission

from the center of the Milky Way galaxy by Jansky [45] and with the advent of radio

engineering and technology, it became clear that radio sources are found scattered all

over the sky. Baade and Minkowski [4] in 1954 identiﬁed one of the brightest celestial

radio sources, the Cygnus A, that was the ﬁrst observed radiogalaxy. A huge advance came in 1963 when Schmidt [75] draw upon observations of the object 3C273 (the meaning of this name is that it is the 273th object in the Third Cambridge Catalogue

(3CR) of radio sources), in the Virgo constellation . The special characteristics of this object was the high luminosity, the existence of a jet and last but not least, its distance that was calculated at a redshift z = 0.158. With other words the 3C273 was 2.8 billion light years away, an amazing great distance for that epoch. A large distance, however, implies an extreme intrinsic luminosity given the measured brightness of these objects. These high luminosities were a problem as no known energy conversion mechanism, such as nuclear fusion, could explain them. Clearly, those discoveries had a tremendous impact in the way astronomers thought about the universe and its limits. The 3C273 named by Schmidt as a ”Star Like Object” and became the

first quasar (Quasi-Stellar Radio Source) ever identified. Hence, things took off very

5 rapidly because this discovery caught astronomers attention. Equally important were and the next steps, such as those of Salpeter [73] and Zeldovich and Novikov [92] at 1964 who speculated that maybe massive black holes powered the strange quasar.

In addition, Salpeter estimated that the mass of the compact center of the quasar was M > 106 ⊙. Another very important paper came from Lynden-Bell [66]. In his study he noted that many galaxies may contain, as he refer, many collapsed quasars. With other words, his idea was that when you see a distant quasar in the universe, they represent a glowing supermassive black hole(SMBH). Lynden-Bell made some estimates about how many SMBH should have grown in the distant universe and compared that with the density of galaxies in the local universe and reasoned that it is quite likely to expect to see what he referred as a ”collapsed old quasar” in the center of many galaxies in the local universe. Indeed, studies today have shown that in fact most massive galaxies have a SMBH in their center. Numerous studies have followed, as the AGN became a topic of widespread study in the astronomical community.

Figure 2.1: Right:A multiwavelength view of the Cygnus A radio-galaxy as captured by Hubble telescope. Image Credit: NASA, ESA, S. Baum and C. O’Dea (RIT), R. Perley and W. Cotton (NRAO/AUI/NSF), and the Hubble Heritage Team (STScI/AURA) . Left:An image montage in optical frequencies of the distant quasar 3C 273. Image Credit: NASA and J. Bahcall (IAS).

6 2.2 AGN Phenomenology

Active Galactic Nuclei show a variety of observational properties, but many of those characteristics must be quantiﬁed by using criteria. In the following session,

we will try to give a brief summary of some AGN classes based in the work of Krolik

[59]. The taxonomy of AGN on classes is, however, quite diﬃcult. As it seems, AGN

constitute a somewhat vaguely deﬁned class of objects. In order to classify these objects, often the famous diagnostic BPT diagrams are used. The BPT diagrams

(named after ”Baldwin, Phillips and Telervich” (Baldwin et al. [5]) are a set of nebular

emission line diagrams used to distinguish between hard and soft radiation. The most

used diagram is the [NII]6584/Ha and [OIII]5007/Hb ﬂux ratio. The next two more commonly-used BPT diagnostics are [SII]6717, 6731/Ha versus [OIII]5007/Hb (BPT-SII) and [OI]6300/Ha versus [OIII]5007/Hb (BPT-OI). These diagrams were studied in numerous work and the dividing lines have been deﬁned and adapted as a function of the ionization models were observations are available (Veilleux and

Osterbrock [86], Osterbrock [71] etc.), and we will present some in what follows.

2.2.1 Seyfert Galaxies

Seyfert galaxies are usually active spiral galaxies found mostly in the low-z uni-

verse. They are recognized by the high surface brightness of their nucleus, that shows unusual emission-lines (Seyfert [77]). These galaxies have a point - like (often called

an as quasar-like also) center whereas the host galaxy is very apparent. We detect

them usually in many wavelengths, from radio waves to even gamma rays. In the

optical wavelengths, the so-called photometric criterion(except the morphological criteria), for nucleus appearance, in order to recognise a Seyfert galaxy is the value of the

absolute B magnitude (calibrated in Vega). A crude criterion to distinguish Seyferts

from QSO’s is the limit in M. When M > −23 is referred to Seyferts, while, when

M < −23 to QSO’s (Schmidt and Green [76]). Also, Seyfert optical spectra show a

7 wide range in ionization level, which is produced by gas ionization. In light of extensive observations, we have adapted the following ”picture” of the

central region of Seyferts. It’s center, is considered to be a compact supermassive

black hole. At a distance of ∼ 1 pc lays the Broad Lines Region (BLR) where

the BLR clouds have very large velocities. Due to the higher density environment,

frequent collisions de-excite atoms and inhibit the born of forbidden emission lines. The forbidden lines are often referred to as being collisionally suppressed. Forbidden

lines are generated when an electron in an excited atom transits from a metastable

state to a lower energy level. Under normal circumstances, (high particle densities),

such an electron would almost immediately leave from the metastable situation by collision, without emitting a photon. But in an environment such as BLR, the time

between collisions allow transitions from higher states. In a nebular environment, the

vast amount of the highly excited atoms drop into these states and from them to the

ground state by forbidden radiation(Eddington [24]).

The emission lines in general are produced by hydrogen ionization and recombi- nation, due to the area’s high luminosity and strong gravitational ﬁeld. Moreover, the line width is caused by high velocity of the material around the nucleus. At ∼

100–1000 pc away from the central engine we observe narrow lines. In this area, velocities are signiﬁcantly lower and the environment is of lower density, compared to BLR region. By the same token, we call this region narrow-line region (NLR)

(Peterson [72]).

The next milestone in the Seyfert history was provided by Khachikian and Weed-

man [53]. Based on observational data, they argued that there are two distinct classes

of Seyfert, based on their emission lines. The Seyfert 1 and 2 classes, also known as Type 1 and 2. As a matter of fact, all Seyfert types have strong narrow emission

lines, but in Type 1 the permitted transition lines are broader than the forbidden.

On the other hand, type 2 spectra are characterized by the equally narrow permitted

8 and forbidden transition lines. To put it diﬀerently, Seyfert 2 galaxies seems to be somehow obscured.

In addition, weak absorption lines are often detectable, since the late-type giant stars in the host galaxy are also observed in both type 1 and type 2 Seyfert spectra.

The absorption lines are weaker than emission, as a result of the dilution of starlight by the non-stellar continuum. In fact, the AGN continuum is usually so weak in Seyfert 2 galaxies that it is very diﬃcult to isolate it from the stellar continuum.

In summary, Type 1 galaxies are characterized by:

• A Broad Line Region

• A Narrow Line Region

• Strong UV - Optical spectra

• X-ray continuum

when Type 2 galaxies show :

• Absence of a Broad Line Region

• A Narrow Line Region

• Absorbed UV-optical spectra

• X-ray continuum

Intermediate Seyfert types have also been recognized, (the Seyfert 1.5 to 1.9 (Os- terbrock [70], Goodrich [36])). The classiﬁcation is based on the optical spectrum. In

brief, numerically larger subclasses having weaker broad-line components, compared

with the narrow lines. For example, in Seyfert 1.9 galaxies, the BLR component is observed only in the Ha emission line, and not in the higher-order Balmer lines. In

Seyfert 1.8 galaxies, the broad components are very weak, but observed at Hβ line

9 and Ha. In the end, in Seyfert 1.5 galaxies the strengths of the broad and narrow components in Hβ are similar.

The physical mechanism that cause all that diﬀerences between Seyfert 1 and

2 is still unknown. However, there are some hints, which some of them we will investigate in detail in the next chapters, that Seyfert 2 galaxies are intrinsically

Seyfert 1, for which we are unable to observe the broad components due to observation of the line of sight or by virtue of interactions between Seyfert galaxies and their neighbours(disturbed material blocking our view to the central region).

The above, constitute the Uniﬁed model, which interprets the diﬀerent classes of

AGN as the same phenomenon viewed in diﬀerent observational angles.

2.2.2 Low Ionization Nuclear Emission-Line Regions(LINERS)

LINERS, are low luminosity AGN which contain ionised gas and powered by

accretion (possibly in a radiatively ineﬃcient regime) onto a SMBH. First deﬁned by Heckman [38], today they constitute a special class of galaxies, the largest AGN-

sub-population, dominating in numbers over the higher luminosity Seyfert galaxies

and quasars. In the local universe, it is estimated that they constitute one-third of

all galaxies brighter than B = 15.5 mag. Further, LINERS often referred to as low- luminosity AGNs. The diﬀerence in the optical spectra is that the continuum emission

of LINER nuclei is weaker, whereas the low-ionization emission lines are stronger. As

the source for ionisation, the low-luminosity AGNs has been widely accepted, with a

potential contribution by shocks. In particular, the main line ratio and continuum

characteristics that describe a LINER galaxy are:

• [OII] 3728/ [OIII] 5007 ≥ 1

• [OI] 6300/ [OIII] 5007 ≥ 1/3

• Strong UV continua and variations

10 Nonetheless, there are evidence indicating that not all LINERS have an active nucleus. Speciﬁcally, the most probable explanation for the excess LINER-like emission is ionisation by evolved stars during the short but very hot and energetic phase known as post-AGB (Yan and Blanton [91] et al. [31]). In Figure 2.2 we show the

BPT diagram and the spectral type classiﬁcation of diﬀerent type of AGN.

Figure 2.2: In this BPT diagram we see the spread of emission-line galaxies from the Sloan Digital Sky Survey (SDSS). This diagram uses 4 strong optical emission lines, [OIII] 5007, [NII] 6583, Ha 6563, and Hb 4861, in order to distinguish galaxies that are dominated by ionization from young stars (green dots, labelled as ”Star-forming Galaxies”), from those that are ionized by an accreting SMBH in their center (Seyfert and LINER galaxies). The solid curve indicates the empirical dividing lines and the dased the theoretical between active galactic nuclei and star-forming (SF) galaxies, based upon the SDSS spectroscopic observations (Kauﬀmann et al. [46], Kewley and A.Dopita [51].

2.2.3 ULIRGS

Ultraluminous infrared galaxies (ULIRGs) are galaxies which are enormously

12 brighter in the infrared regime than a typical galaxy(Lir > 10 ⊙). This type of AGN was discovered by the InfraRed Astronomy Satellite (IRAS), which was launched in

1983. The ﬁrst mention of them appeared in the publication Houck et al. [41]. A galaxy can emit in the infrared because of sources like stars, interstellar gas, and

11 dust. Emission from atoms and molecules in interstellar gas has a little contribution in the infrared emission of galaxies. The main bulge of infrared radiation beyond 3 microns is thermal emission from heated dust by starforming activity.

However, ULIRG galaxies in the local Universe are rare, and they contribute only ∼ 6% of the total infrared luminosity energy density (Soifer and Neugebauer

[82]). Surprising are the result of the study of Sanders et al. [74], which show that 95% of ULIRGS are mergers, as there is direct evidence for powerful galaxy-scale winds in most ULIRGs (Veilleux and Osterbrock [86]). Moreover, the spectral energy distributions of IRAS ULIRGS were reviewed and compared with those of QSOs and

Blazars.

2.2.4 Radio Galaxies

Radiogalaxies are intense sources of radio emission and usually they are associated with giant elliptical galaxies. The radio luminosity of a strong radio galaxy can reach the 1037–1039 watts, an energy by far greater than the radio emission of an ordinary galaxy (1033 watts). In fact, most of the radio emission from such a galaxy comes from two lobes, often located symmetrically around the visible galaxy. Observational studies have shown that the nature of radio emission is due to clouds of plasma ejected at some time in the past from the central region of the galaxy. Today, it is a common knowledge that jets are an eﬃcient way of dumping angular momentum and in a radio galaxy it can extend up to several Mpcs. High resolution images have shown that many radio galaxies also possess a compact central radio source from which a simple, or a pair of oppositely directed jets, of radio emitting material, emerge pointing outwards towards the distant lobes. Additionally, perturbations injected though the jet nozzle propagate upstream through the steady-state ﬂow and can create traveling, standing and even backwards moving shocks. The jets appear to be streams of highly energetic electrons that have been accelerated with extreme velocities, reaching large

12 fractions of the speed of light(Davis and Conway [21])and also radiate synchrotron radiation .

The spectrum of a radio galaxy in the optical frequencies revealed that radio galaxies can be also separated into Narrow Line Radio Galaxies (NLRGs) and Broad

Line Radio Galaxies (BLRGs), similar to Seyfert galaxies.

Figure 2.3: Stacked 43 GHz image from the radio galaxy Gygnus A, observed with the Global VLBI at 7mm wavelengths. This resolution corresponds to a linear scale of approximate 400 Schwarzschild radii. (Boccardi et al. [11])

Moreover,([32]) introduced a new classification method for radio galaxies, based on the morphology of their jets. They used the ratio between the distance of the positions of the brightest radio emission on each side of the central source to the total size of the radio structure. Objects in which the FRratio < 0.5, their low brightness regions is further from the central galaxy whereas have broad jets which ending in plumes, belong to class I. FR II class, comprises luminous radio sources with hotspots in their lobes at distances from the centre is defined by a ratio above 0.5. Other significant features of FR II radio galaxies are their high luminosity, the fact that the jets are also brighter than those of FR-I galaxies, but on the other hand their lobes are much fainter. Also, the jets are often one-sided and in general, FR II are associated with galaxies that appear regular, except that they have nuclear and extended emission line regions.

13 2.2.5 Quasars or QSO’s

Quasars or OSO’s (for Quasi-Stellar Object) are the most luminous AGN and typically they are very distant(Highest QSO reach z = 7.085 et al. [30]). Their appearance in optical frequencies is point like with absolute magnitude M < 23mag.

Furthermore, some quasars are detectable in radio frequencies. For this reason there are two classes, based on radio emission, the radio loud type I quasars and the radio quiet type II. Many studies have shown that 90% of optically detected OSO’s are type II (Kellermann et al. [48]). Optically selected samples, however, have shown that many quasars are radio-quiet. Equally important is their distinct appearance in the UV spectrum. For instance, even in the early days of their discovery it was notable that quasars appeared to be bluish objects compared to regular galaxies. The physical explanation is the famous big blue bump. This bump is due to thermal emission from an accretion disk around a massive black hole (Shields and Searle [78], Malkan and

Sargent [67]), and it was suggested that its energy peak lies in the unobserved extreme ultraviolet.

In the ﬁnal analysis, some of the most important characteristics of the quasars are :

• Nuclei that appear star like in optical images

• Large redsifts

• Absolute magnitude M < 23mag

• Radio emission in some cases

• Variable continuum ﬂux

• Strong UV component

• Broad emission lines

14 Figure 2.4: This is the spectra of the ﬁrst quasar in history. As we can see, the emission lines are shifted because of Doppler Eﬀect(∆λ = u/c, where λ is the wavelength of the line and c the speed of light and u ≪ c).

2.2.6 Blazars

Blazars are compact objects, associated with an active SMBH at the center of an

elliptical galaxy. They are separated into two categories, BL Lacerta galaxies (BL

Lacs) and Optically Violent Variables Quasars(OVV QSOs). Both are highly variable and polarized at visible wavelengths, radio emitters and gamma ray sources. They

present absence of emission lines above the continum. In particular, owing to this fact,

French and Miller [34] tried to estimate their distance measuring the neighbouring galaxie’s emission lines redswift. BL Lacertae, were originally recognised as variable stars, due to the extreme luminosity variability (even up 10 mag during a single day). The vast majority of their host galaxies are elliptical galaxies. Also, these objects appear to have a jet, characterized often by ultrarelativistic velocities which is roughly aligne with the line of sight. On the other hand, the optical violently variable (OVV) quasars are similar in many aspects. The basic diﬀerence between OVVs and BL Lacs is that OVVs emit strongly in the UV regime and show optical emission lines([87]). OVVs also have higher luminosities and higher inferred Lorentz

15 factors than BL Lacs.

2.3 The Uniﬁcation Paradigm

Numerous models have been suggested with the intention to unify all the AGN classes during the years. The common goal of those models was to attribute the wide range of observational properties to the minimum parameters. For instance,

Blandford and Konigl [10] and Antonucci [2] point out that compact and extended radio sources are the same kind of objects, observed by diﬀerent angles(angles between their jet and the line of sight). The next step was the uniﬁcation of all Seyfert types as a result of their orientation. Antonucci and Miller [1], stated that the polarization spectra of some Sy2 galaxies contained broad emission lines like those seen in Sy

1. A convincing explanation of this phenomenon was that both types of AGN’s have an axisymmetric central engine, surrounded by an accretion disk and a thick dusty torus(on scales of 1–100 pc), since the most common cause of polarization is the scattering of light by either dust or electrons. The UV–optical continuum emission is supposed to arise primarily in the accretion disk. But, the result of the existence of a torus was the obscuration of the radiation coming from the broad line region. According to this scenario, if we observe the galaxy in straight line to the nucleus, we are able to detect all the spectral features, like the broad emission lines. Looking instead the galaxy edge-on, the torus blocks the view of the BLR clouds and only the NLR is visible. The next step was taken by Urry and Padovani [85], when they published a review about the explanation of the uniﬁcation of radio-loud

AGN by orientation. Specifically, they said that BL Lac objects are those FR I, only viewed at small angle. Furthermore, Barthel [7] claimed that flat-spectrum radio quasars (FSRQ) are the same class of objects as FR II radio galaxies, only closely oriented to our line of sight. Additionally, these models apart from the orientation of obscuring torus suggest that relativistic effects are present as well, to account for

16 Table 2.1: The Grand Uniﬁcation Scheme based in radio emission

Oriantation Face On Edge On Radio Properties Seyfert 2 Seyfet 1 Radio Quiet QSO ULIRG NLRG BLRG Radio Loud FR I BL Lac FR II Quasar OVV the characteristics of powerful blazars. A unification scheme stems also in X-rays radiation. Observational data have shown that Seyfert 1 are detectable in soft X-rays whereas Seyfert 2 are absorbed (Awaki et al. [3]). To summarize, the current unification paradigm properties depend on several fundamental aspects of the nucleus. The first parameter is the mass of the SMBH, which obviously is the strength of the gravitational engine that powers the nuclear activity.

The second parameter is the accretion rate, the rate at which gas fuels the nuclear activity. Efficiency of the energy processing in AGN’s is very high, reaching the 10 or 30 % of the mc2, many times higher than the efficiency of nuclear fusion in stars. The third parameter is the orientation. Due to spinning of the black hole, jets are created which consists of relativistic material. The way an active galaxy looks depends on the orientation of the line of sight with respect to the dust torus. The view looking down the jet or so is entirely different than the one at which the jet is at a right angle with respect to line of sight. The forth aspect is the nature of the host galaxy itself.

As a consequence active galaxies are usually spirals, having a larger gas supply than an active galaxy hosted in an elliptical host. These four properties can be combined to describe a uniﬁed model of all diﬀerent types of active galaxies.

17 Figure 2.5: A uniﬁed model of AGNs. The upper right part of the drawing corresponds to high-power sources with the jet emerging from an open torus, the left upper part, represent the low-power sources with the jet emerging from a closed torus. Diﬀerent morphologies are produced by the orientation of the observer with respect to the obscuring torus. Credit: Beckmann and Shrader [8].

2.4 Beyond the Uniﬁed Model

The Uniﬁcation Paradigm of AGN states that all the observed diﬀerences between active galaxies are due to the orientation of the torus with respect to the line of sight.

The key element of this uniﬁcation model is the obscuring medium, which is pictured as a toroidal structure on a parsec scale. However, since the model was proposed a large amount of observational facts has been accumulating which indicate that some observable phenomena cannot be explained by the simplest uniﬁcation model. It has been found that the local environment of Sy1 and Sy2 (e.g., Villarroel and Korn [88],

Dultzin et al. [23], Krongold et al. [61], Koulouridis [55]) exhibit diﬀerences. For ex-

18 ample, a tree dimensional study of a properly selected sample showed that the local environment of Sy1 and Sy2 ( Koulouridis et al. [58], Koulouridis et al. [56], Khabi-

boulline et al. [52] and references therein) it’s not the same. Speciﬁcally, they found

that there are indications of a higher ionisation of the closest neighbors of Sy2s with

respect to neighbors of Sy1s, a fact hinting towards a stronger interaction in the vicin-

ity of Sy2’s. That ﬁnding introduce a possible evolution scenario which states that, a gravitationally disturbed Seyfert 1 galaxy could end as a Seyfert 2. Another one

important example comes from Elvis and Nicastro [28], when they studied a sample

of 25 Seyfert 2 galaxies and they found that 90% of sample’s galaxies show signiﬁcant

variations of their X-ray absorption column density. This cannot be explained by absorption from the simple torus model. Equally important are the clues regarding the

clustering of diﬀerent types of AGN, with obscured and unobstructed AGN showing

diﬀerent clustering patterns (for example, Elyiv et al. [29],Bianchi et al. [9]). Such

observational facts have also been explained by an evolutionary component aﬀecting

the phenomenology of AGN ( Koulouridis [55]). Finally it is still not fully understood the AGN triggering mechanism and the AGN evolution, and duration of their nuclear activity. There are also many proposed scenarios about the possible evolution of an

AGN, the linking with the SMBH as well as the triggering mechanism which activate it and the co-evolution of the host galaxy (Merloni and Heinz [68], Caplar et al. [16]).

Although, Cheng et al. [17], Koulouridis [55] and many others, suggest that close interactions are a possible triggering mechanism of AGN activity, as close encounters

appear capable of activating a sequence where an absorption line galaxy ﬁrst becomes

a starburst, then a Sy2, and ﬁnally a Sy1. This evolutionary scenario is supported by

many studies (Ballantyne et al. [6], Treister et al. [84], Hasinger [37] etc) which shown that unobscured AGNs trace the same evolution as obscured AGN’s, a behaviour that

it is not explained by the uniﬁcation paradigm. Additionally, very extended halos

which are seen usually around the radio galaxies, are detected around some of the

19 radio quiet sources as well. Furthermore, a recent discovery revealed for the ﬁrst time a quasar changing AGN type within 10 years which was transformed to a Seyfert 1.9.

(LaMassa et al. [62])

Figure 2.6: Simple schematic of galaxy and AGN evolution (Hickox et al. [39])

20 CHAPTER III

Research Goal and Methodology

In Chapter II we discussed the theoretical background regarding the AGN phenomenology. In this current chapter, we want to emphasise in an important aspect that regards the interactions between AGN and their neighbours through tidal forces and how this phenomenon can contribute to the activity of a supermassive black hole and even in the evolution of galaxies.

3.1 Torus

As we described in the previous chapters, the dust torus, which is located very

close to the central area and around to the supermassive black hole, plays a crucial

role in the identification and classification of AGN. Specifically, we can describe this component of an AGN as ”dusty clouds”, which are individually optically thick, in a

toroidal structure” (Krolik and C. [60]). The clumpy torus in general, is characterised by two fundamental properties:

• Anisotropic obscuration of the SMBH, so an active galaxy viewed face-on is

recognised as type 1 and when viewed edge-on as type 2.

• The re-emission of radiation in the infrared band by dust. This emission gave us a unique chance to estimate how extended the torus is radically.

21 The vast amount of observations till today supports the idea that Seyfert 1 and 2 galaxies are the same type of objects, observed at different angles (Hien [40]). Nev- ertheless, there are many studies which find that other factors can play an important role in the appearance of an AGN. For example, there are indications that AGN probably evolve over time, starting from a deeply-buried active nucleus which eventually consumes or expels the material that surrounds it (Sanders et al. [74]). Also, it has been suggested Siebenmorgen et al. [80]that the material of the torus is found in two phases. According to this study, dust in the AGN torus is distributed in a clumpy medium even in the polar regions around the SMBH, in a radius of 50 pc, or in a homogeneous disk or as a combination of the two as can bee seen in the following figure.

Figure 3.1: A representation of the 2 phase torus (a homogeneous disk or a clumpy medium) and of the host galaxy structure together with the thermal distribution around the central engine (as presented in the work of Siebenmorgen et al. [80]).

In conclusion, we can see that clumpiness in the dust torus can alter the one-to- one relation between orientation and AGN type as supported by the uniﬁed model

(Elitzur [25], Buchanan et al. [14], Levenson et al. [64]). As a consequence even if

22 an AGN is viewed ”face-on”, it could be classiﬁed as a type 2 if even a single cloud happens to block the broad line region (BLR) from the view. The clumpiness also

implies that the dust temperature is not a simple function of distance from the AGN.

Diﬀerent clouds may have simultaneously a hot and a cool side, that which is directly

illuminated or that facing away from the accretion disk, respectively (Buchanan et al.

[14], Levenson et al. [64]).

3.2 Ha Balmer Line

It is well known that 90% of the universe consists by hydrogen, an element that can be found in many stages depending on the environmental conditions. The emission

spectrum of the atomic hydrogen is separated in spectral series, in wavelengths given

by the Rydberg formula(Bohr [13]). These observed spectral lines are due to the

electron making transitions between two energy levels in the atom, as we can see in ﬁgure 3.2. One of the most famous series in astronomy is the Balmer group. Four of these lines are almost in the visible part of the spectrum, with wavelengths longer than 4000 Åand shorter than 7000 Å. Wavelengths of these lines are given in the following table.

Table 3.1: The most used in astronomy lines of Balmer Series.

Line nf ni Wavelength (Å)

Hα 2 3 6563

Hβ 2 4 4861

Hγ 2 5 4340

Hδ 2 6 4102

The Ha line at 6563 Åhas been one of the most useful lines in optical astronomy,

23 indicating regions which have been photoionized by a nearby star. Young, massive stars produce large amounts of ionising photons and this leads to the ionization of the surrounding gas. Thus, observing Ha emission in a region indicates that hydrogen is being ionized there. Astronomers use observations in this wavelength as an eﬀective probe of star formation, where surrounding gas is continually ionized. This is why this line represent the most traditional starformation indicator (Kennicutt [50]).

Figure 3.2: Electron transitions of the hydrogen atom and the wavelength of the each emission line that stems from.

Narrow Band Filters

Narrow-band ﬁlters are designed to capture speciﬁc range of wavelengths. These

ﬁlters are used mainly in observations of a large class of celestial objects known as emission nebulae. The name of these objects arises from the fact that they emit their own radiation. This category includes regions like common nebulae but also supernova remnants and starforming regions. The characteristic that all emission nebulae have in common is that they are composed of gases, and these gases are ionised and emit radiation.

The primary advantages of the narrowband imaging is the ability to depict with

24 great detail specific physical processes and isolate the light given by specific kinds of gases. Since such filters allow less starlight to pass, its is necessary to obtain longer

exposures which however are not saturated due to the possible presence of bright

stars in your frame.

3.3 Galaxy-Galaxy Interactions

Even though the typical distance between galaxies is huge, galaxies cluster and

some will come close to their neighbors at some point in their lives. When galaxies

interact, collide or merge, there are labeled as interacting galaxy pairs. Such interac-

tions play an important role in the evolution of a galaxy, since in such circumstances gases are compressed, mix together, and eventually they set oﬀ rapid star formation.

These processes also disrupt the morphology and structure of the two galaxies. Some-

times, such interactions will cause the two galaxies to merge into a single, massive

galaxy. The causes of such interactions are mainly gravitationally driven. The resulting eﬀects on the galaxies are many, and are related mostly to star-formation and

AGN activity. A key role to this connection is the angular momentum conservation

and the tidal forces(Xuan et al. [90]). A disruptive tidal ﬁeld naturally tends to ex- pel material from disk galaxies. It has been shown that even an isolated galaxy in order to minimize its total energy tends to transfer its angular momentum outwards (Lynden and Kalnajs [65]). In eﬀect, in the interacting cases we observe in many

occasions large tidal tails. But, the tidal ﬁeld it is not the only mechanism respon-

sible for the formation of tidal tails. Tidal forces cannot fully explain the important

phenomenon of central gas inflows. In fact, this motion is mostly driven by gravity torques. Specifically, the tidal field from a companion breaks the symmetry of the

gravitational potential. This cause a response of the disk material which is cold gas

(molecular clouds). The gas can form a pair of spiral arms, like presented for M51 by

Dobbs et al. [22]. At this point we have to note that these spiral structures do not

25 require an interaction for them to form (Elmegreen and Thomasson [27]). The gas motion during interactions is often more complex than in a pair of spiral arms. Al- though, a main characteristic remain the same when there is corotation in the galaxy.

Inside the corotation radius, the gas tentd to concentrate on the leading side of the potential well. Outside of the corotation it concentrates on the trailing side. A corotation in general is located through a few kpc of radius in the disk. An illustration of all above can be seen in ﬁgure 3.3.

Figure 3.3: Inside the corotation, the gas undergoes negative gravity torques, and looses angular momentum in a rapid central gas inﬂow towards the central area. In the outer disk, gas would gain angular momentum and ﬂy out to intergalactic space in long tidal tails.

Astronomers have been debating since more than 20 years whether the environment of active galactic nuclei (AGN) or starburst galaxies is different from that of inactive galaxies. This idea stems from the observational indications that nuclear activity, as well as starburst activity in galaxies, is triggered by galaxy interactions and by the fact that the environment of active and non-active galaxies is different for different types (Hutchings et al. [43], Dahari [20] Keel et al. [47] Bushouse and Werne

[15], Koulouridis et al. [56]and references therein). Also, several studies ( Koulouridis et al. [58] ,Shlosman [79], Cheng et al. [18], Ellison et al. [26], Ideue et al. [44]and references therein) have shown that interactions are connected with the central SMBH

26 activity. As we mentioned before, the concept is that molecular clouds are driven straight to the central area and often this has as the result of the creation of a cir-

cumnuclear starbust. Additionally, it is possible that material fall into the SMBH,

creating with this procedure an obscured AGN galaxy (Koulouridis et al. [58]). Ob-

servations of this stage would show only a starburst event. Later, the central area

will be revealed and we will observe a Sy2 galaxy (obscured by molecular clouds from all view points). Finally, a ”Sy1” stage would appear as a result of the ﬂattening

of the molecular clouds into a disk. Also, there are indications that the number of

close neighbours around a Sy1 is smaller in comparison with a Sy2 galaxy (Villarroel

and Korn [89], Koulouridis et al. [56] and references therein) and that the neighbours around Sy2 seems systematically more ionized than those around Sy 1 (Koulouridis

et al. [57]). Finally, recently suggested by Ghisellini and Sbarrato [35] the scenario

of the creation of a dark bubble of material around the active galaxies could change

the observed characteristics.

3.4 Our Aim

In the previous paragraph (3.3) we present the main evidence, observations and hypotheses related to the activity of AGN galaxies and their appearance. These indications have to be substantiated further in order to understand in which way the environment affects the AGN phenomenology. This paper, focuses exactly on such issues, on deepening our understanding of these phenomena. The approach we have decided to follow is to use narrow band imaging of a local sample of AGN galaxies and their neighbours. Specifically, this thesis is a pilot project which aims in studying Sy1s and Sy2s which have close companions by using CCD observations in the narrow-band Ha filter.

We attempt to ﬁnd indications of stronger interactions of Sy2s with their neareast neighbour.

27 The Unified model can explain most of the observable differences between the AGN. Although there are many details to be examined. The model that we are investigating suggest that the interactions between galaxies lead great amount of gas and dust in the nuclear area, due to angular momentum conservation. reation of a cocoon around the nuclei and camouflage Seyfert 1 galaxies to Therefore, the indications of interactions that we are aiming to find are extended regions of Ha line emission in the regions between the AGN and their closest neighbour (since strong interactions can induce star formation). The idea which we are attempting to test is that if there is strong interaction then gas and dust will not only be swept away from the galaxies in their intergalactic regions but due to angular momentum conservation, quantities of gas and dust will fall towards the nucleus, creating a ”cocoon” of material and thus possibly masquerading Sy1 to appear as Sy2.

It is worth mentioning that there are some signiﬁcant new indications supporting the hypothesis that AGN can change their appearance and that there is a signiﬁcant contribution of the ”cocoon” of material around the central area in the AGN phenomenology and not only in optical frequencies but also in radio waves. A recent study (LaMassa et al. [62]) report that the quasar J015957.64+003310 has transi- tioned from a Type 1 to a Type 1.9 AGN between 2000 and 2010.

28 CHAPTER IV

Observations

After the above introduction in some of the most basic concepts related to AGN, which are required for better understanding of the rest of the paper, in this chapter

we are going to give a description of our observations and the methodology we have

followed.

4.1 The Telescope

Helmos Observatory (6.11) is located on mount Helmos (Neraidorachi Peak) in

the Northern Peloponnese at an altitude of 2340 m (22o 11 46 East, 37o 59 04

North), 130 km west of Athens, about 15 km from the city of Kalavryta and it is the largest optical telescope in the country. The place where it is located is one of the darkest areas in Europe. The telescope was constructed by the company Carl Zeiss and it works since the summer of 2007. It’s ﬁeld of view is approximate 10 arcminutes in each side and it can reach an observational limit magnitude of V ≈ 24 in an hour of exposure in a dark night.

29 Figure 4.1: The Aristarchos telescope of the Helmos Observatory. The telescope has a Ritchey-Cretien optical system with a primary mirror of 2.280m in diameter whereas its focal ratio is f/8 and its focal length reaches the 17.714 meters. Photo credits: Theofanis Matsopoulos.

4.2 Instrumentation

In order to acquire our data we used a Charged Couple Device (CCD) camera, model Princeton Instruments VersArray 1024B. The type of the CCD sensor is a SITeAB sensor, back-illuminated, Grade 1 and it provides to the user an 1024 x

1024 pixels with a 24 µm2 size for each pixel. This size corresponds to 0.28 ” x 0.28” observational ﬁeld. The total ﬁeld of view with this equipment is 4.8 x 4.8 arcminutes.

It also worth to mention that a cooling system with liquid nitrogen is used. For our observations we used the Continuum Red ﬁlter which has a diameter of 64 mm and it’s transmission factor is 77.57 % .

30 4.3 Preparation and Observations

4.3.0.1 Preparation

For the observing procedure, we had prepared a detailed plan for every night.

First of all, we had chosen our main AGN targets based on several factors. The most important of which are the following:

• We had chosen our sample from the paper of Koulouridis et al. [57] and others

because they refereed to AGN galaxies with close and in most cases gravitational

interacting neighbours. We chose an equal number of Sy1 and Sy2 galaxies.

• The neighbours were chosen considering as criterion their angular distance from the AGN. We wanted of course both AGN and the neighbours to be in our ﬁeld

of view which was 4.8 x 4.8 arcminutes. The selection was done with the usage

of the SDSS Navigation Tool.

• The visibility of our targets was another key point. Using the star altitudes on

line calculator, which is provided by the Isaac Newton Group of Telescopes, we selected the galaxies with the longer celestial path. As a result, the galaxies

whose maximum altitude is below 33 degrees were excluded. We have also

choose an already known disturbed pair, the NGC 7682 - NGC 7679, as clearly

seen in SDSS broad band catalogue, as a control sample. It is also worth to mention that we have calculated the proper coordinates (for the guiding system)

in order to include both the AGN and the neighbour in the ﬁeld.

• A great number of standard stars was chosen from the Landolt catalogue, at

diﬀerent airmasses (we will discus about this issue in the next chapter ref.

standard and airmass) in order calibrate the measured ﬂuxes. The sorting of stars was based on how strongly they emit in Ha line as well as their visibility,

as in the case of our galaxy targets.

31 • We selected the proper ﬁlter for our observations. The galaxy redshift has a crucial role in observations, so we calculate the wavelength at which the Ha line

will be shifted at the galaxies’ redshift according to:

λ − λ z = o (4.1) λo

where z is the target redshift, λ is the measured wavelength sift and λo is the true Ha wavelength. Our target galaxies have such a redshift that the continuum red

ﬁlter at 6680 Å, with 100Åbandwidth, is the best ﬁlter to be used from those

available at Aristarchus.

Finally, in order to remove the instrumental noise and systematics from our data we obtained some necessary science frames like ﬂat-ﬁelds and bias. We had prepared a timetable for the observations of each night. Before the observations we were familiarized with all the equipment and we took part in the procedure of camera cooling using liquid hydrogen. Also, during the nights we continuously checked for systematic errors while we were taking the data. For instance, we checked our images to see if the exposures were not long enough to reach saturation (which happens at 65.000 counts/pixel), especially for our standard star images.

Of course, as it is well known weather conditions is an unexpected factor that can ruin ones’ observations and indeed out of the three observational runs, 15-18 August

2015, 17-19 October 2015 and 7-9 November 2015, only in our last run the weather conditions allowed us to have successful observations. It is worth to mention that in order to achieve high quality science images, in order to avoid any guiding problems or other problems related with cloudiness, we took many exposures of almost 30 minutes each.

32 Figure 4.2: An artistic photograph of the observatory during a rare atmospheric phenomenon (sundog and circumnuclear arc) taken by the author.

4.3.1 Observations

As we mentioned before, our pilot sample is a small number of local Seyfert 1 and

Seyfert 2 galaxies from the original Koulouridis et. al. sample. We have been awarded with 3 observational runs with the 2.3m Aristarchos telescope, of 3 days each, but unfortunately two runs were wiped out by bad weather. The observed objects are shown in the next table.

33 Table 4.1: Observational sample of galaxy pairs in Ha line. Considered the galaxy redshifts, we used the Continium Red ﬁlter in 6680 Åwith an BW 100 Å.

Name R.A (h:m:s) Dec (d:m:s) z Type OMAP S Date Exp (s) Airmass Humidity NGC 7469 23 13 15.5 +08 52 26 0.0162 Sy1 14.48 07/11/15 3600 1.278 44% Neighbor 1 23 03 18 +08 53 37 0.0156 15.58 NGC 863 02 14 33.5 -00 46 00 0.027 Sy1 14.58 07/11/15 4800 1.28 40% Neighbor 1 02 14 29.3 -00 46 05 0.027 18.25 MRK 612 03 30 40.9 -03 08 16 0.027 Sy2 15.78 07/11/15 3000 1.455 34% Neighbor 1 03 30 42.3 -03 09 49 0.0205 16.13 NGC 1019 02 38 27.2 +01 54 31 0.0205 Sy1 15.02 08/11/15 1200 1.454 62% Neighbor 2 02 38 25.4 +01 58 07 0.0242 16.28 NGC 3786 11 39 42.8 +31 54 33 0.0091 Sy2 13.88 08/11/15 3600 1.627 60% Neighbor 1 11 39 44.6 +31 55 52 0.0085 13.53 NGC 7682 23 29 03.8 +03 31 59 0.0171 Sy2 14.88 08/11/15 1200 1.28 50%

34 Neighbor 1 23 28 46.6 +03 30 41 0.0171 14.64 UGC 12138 22 40 17 +08 03 12 0.025 Sy2 15.93 08/11/15 4800 1.194 41% Neighbor 1 22 40 11 +07 59 59 0.0236 18.77 IRAS 00160-0719 00 18 35.9 -07 02 57 0.0187 Sy2 15.73 09/11/15 4800 1.42 13 % Neighbor 1 00 18 33.3 -06 58 54 0.0173 17.8 NGC 1194 03 03 48.4 -01 06 09 0.0134 Sy1 15.38 09/11/15 4800 1.337 12 % Neighbor 1 03 03 41.2 -01 04 25 0.014 16.99 NGC 1241 03 11 14.8 -08 55 15 0.0135 Sy2 13.56 09/11/15 2400 1.711 40% Neighbor 1 03 11 19.3 -08 54 09 0.0125 15.41 NGC 1320 03 24 48.8 -03 02 26 0.009 Sy2 14.59 09/11/15 3000 1.72 41% Neighbor 1 03 24 48.6 -03 00 56 0.0095 15.07 NGC 3786 11 39 42.8 +31 54 33 0.0091 Sy2 13.88 09/11/15 2400 1.744 51% Neighbor 1 11 39 44.6 +31 55 52 0.0085 13.53 NGC 7682 23 29 03.8 +03 31 59 0.0171 Sy2 14.88 09/11/15 4800 1.234 14% Neighbor 1 23 28 46.6 +03 30 41 0.0171 14.64

Note: OMAP S refer to the apparent magnitude. Exposure times are the sum of more than one exposures for the same pair. CHAPTER V

Data Analysis

In this chapter we will discuss about the data analysis methods we choose to follow. We studied two applications of photometry, distinguished by the kind of object that are excited in the sky each time. For the sake of concreteness we will mention the basics of the photometric methods and CCDs.

5.1 Background

In astronomy, the mainly usage of Charge-Coupled Devices (CCD) is to acquire digital images of celestial objects with the intention to study their behaviour, evolution and interactions. Nowadays, CCD sensors are very popular because of their characteristic high ratio of the number of photons which can be detected compared to the total number received. A CCD sensor is a silicon chip that converts an image to an electrical signal. The chip contains a grid of pixels which are acting as photon-counters. We can imagine them as tiny buckets which collect light, as we can see in the ﬁgure 5.4. When a photon hits this raw of pixels, it is transformed into an electron-hole pair by the underlying semiconductors due to photoelectric eﬀect.

During an exposure, the pixels are ﬁled by photons. Then, the transferring of the pixels must be converted into a digital signal. The exact process diﬀers from CCD to CCD, but the basic steps are the same. The signal in each pixel is applied to

35 a circuit the so-called Analog-to-Digital Converter. This device converts the analog signal into a digital value. Then, each pixel value read out of the camera and sent it to the computer where it is saved as a ﬁle.

Figure 5.1: An illustration of the collecting procedure of a typical CCD sensor. The photons ﬁll the pixels, which are converted in order to reach the computer as a digital signal.

However, in order to get the raw data, the instrumental contribution must be removed. Read out noise, thermal current and quantum eﬃciency can change the linearity of the CCD and give us an disoriented counts number.

Thus, data must be corrected before the analysis. This can be done with calibration frames. So, before the observation a number of science frames must be captured.

These frames are the bias, ﬂats and dark frames.

36 5.2 Data Reduction

5.2.1 Dark Noise

Each frame we obtain from CCD cameras contains a background noise, which is caused by the high speed of some electrons, the so-called thermal electrons). These electrons are recognized by the camera as source photons. So, in order to reduce the electrons spread which cause the well known dark noise, we cool the sensor with liquid hydrogen at the −100o C. As a consequence, we didn’t obtain dark frames for the purpose of this paper.

5.2.2 Bias Subtraction

Bias corrections is the first reduction step that should be performed. The bias level is an offset of a few hundred counts which exists to the ccd before the start of the exposure. In practise, it is a zero-length exposure which show us the underlying structure in the CCD image. Every observing night should obtained at least 10 bias frames before and after the observations. These frames are stacked together to create the master bias frame. This master bias must be subtracted from all the data. By averaging 10 bias frames, a master bias was created and subtracted from the images in order to remove any remaining bias structures. In CCDs the bias is not constant in every pixel. It can also vary in time and across regions of the CCD chip. The effect of the bias is the presence of additive noise and it must be corrected by determining the value of the bias in order to subtract it from each pixel in an image .

37 Figure 5.2: Left: A print screen of the adjustments in Aristarchos remote control environment in order to obtain bias. Right: A master bias frame of our observations.

5.2.3 Flat Field Correction

Another necessary step to calibrate CCD images is the division with the ﬂat frames. Pixels have not the same response at the same amount of receiving photons.

Furthermore, during it’s path to the sensor the photon is reflected by the mirror surfaces, which are usually covered with dust particles. Dust particles cause donut shapes in the image, as can be seen in figure /ref . The optical elements of the telescope can also cause vignetting which is obvious on the edge of our frames. These effects can be eliminated by the flat fielding procedure. This procedure requires the obtain of several frames (usually 10 frames for each filter) of a featureless, uniform source, such as the twilight sky or a dome projector screen. The focus should be the same with this used for the object frames.

The twilight sky were used in this project. A set of a number of ﬂat frames were taken during an observing session for each ﬁlter that were used.

A flat field shows the minor pixel variations, as well as all the defects in the optical train. Also, before the reduction procedure the flat frames must be normalized to unity. In this project we determined the mean intensity level of each flat using IRAF

CCD reduction software (using the command imstat). Then, we divided each ﬂat with it’s mean value. Finally, we combined these frames to one master frame and we

38 devided this frame with all the pictures. The flat field frames were combined using the imcombine command. The division of each pixel in the image by the flat have performed using the imarith command. Imarith is part of the package, images.imutil.

Figure 5.3: A snapshot of the CCD cooling procedure using liquid hydrogen in Aristar- chos telescope.

Figure 5.4: A typical appearance of a master ﬂat. This frame is a combination of 7 frames before our observations and 7 after. We create a master ﬂat for each and every night.

39 5.2.4 Fixing the Problems

5.2.4.1 Bad Columns

More often than not, CCD frames have a nunber of rows or columns of high signal.

These series of pixels are not exposed to the light, which create an a bad column eﬀect

in the picture. In order to correct this error we can apply a bad pixel mask which can be made by IRAF. We performed this with the ﬁxpix task. This task ﬁxes bad

pixels by linear interpolation from nearby pixels.

5.2.4.2 Negative Pixel Values

Negative pixel values appears when a pixel is saturated with photons. This eﬀect

occurs often in the center of very bright stars. A second parameter which can cause

this alteration is the CCD temperature. When the camera starts to heating during

the observation’s night, some pixels with negative value appears in the picture. In our data, negative pixel values were distributed randomly. In order to correct the

pixels values we converted the data from signed to unsigned integers with the IRAF

task rﬁts.

5.2.5 Cosmic Rays Cleaning

High energy cosmic particles hits in the sensor are really common in long expo-

sures. These particles lose their energy by knocking sensor’s atoms. That allows to

many electrons to be free and as a result, a bright spot show up. We can easily spot a cosmic ray hitting because they are very bright and sparsely scattered over the

image. We use the IRAF task lacosim for cleaning our data. This task cosmic rays are detected by the deviation of a given pixel by a certain threshold over the variation in the neighboring pixels. The value of this pixel is then replaced with some value calculated from the neighboring pixels.

40 Figure 5.5: At left we can see a single ”dirty” image of the NGC 1241 galaxy and its neighbour. At right we can see a combination of three exposures, cleaned from CCD noise and cosmic events.

5.2.6 Galaxy Extinction

Observations in broad and in narrow ﬁlters of extragalactic objects are faced with the galaxy extinction problem. The extinction is the sum of two physical processes.

The absorption and scattering by the Milky Way dust particles. The size of some grains are comparable with the incident wavelength and as a result there is a scattering peak in the eﬃciency. We cleaned our data using the SDSS galaxy extinction value for the r band of wavelength which corresponds better to Ha ﬁlter we have used.

5.2.7 AB Magnitude

In astronomical observations, due to measuring the ﬂux of an object ordinarily a system of measuring stellar brightness called magnitude system it is used. The idea of classifying stars in a magnitude system was introduced by the ancient Greek scientist Hipparchus. In his magnitude system, the brightest stars are grouped as

ﬁrst magnitude category, a little bit fainter stars as second magnitude and ﬁnally, the stars that eye could see, sixth magnitude. In fact, we know that there are two

41 magnitudes of a celestial target. One express the brightness of a star as observed from Earth and it is called apparent magnitude (m). The other which express a measure

of the total amount of light emitted by the object in every direction is labelled as

absolute magnitude (M). The apparent magnitude of a star corresponds to a ﬂux

value whereas the absolute magnitude of a star is equivalent to its luminosity. In

order to quantify brightness, we use the ﬂux value, which is deﬁned as the rate of the energy which is collected by a telescope with an 1 m2 aperture. Flux measured in

units of J/s/m2 . Based on that idea, today there are numerous magnitude systems.

Each of these, use a speciﬁc star ﬂux or apparent magnitude as a comparison star

in order to express any other object as a fraction of the magnitude of this star. The general concept is that the apparent magnitude of two objects, one of which is used

as a reference, their magnitudes measured from Earth in units of ﬂux, will be related

with the equation:

( ) F1 m1 − mref = −2.5log10 (5.1) Fref

where m1 and mref are the apparent magnitudes of the stars and F1, Fref their ﬂuxes. For example the Johnson System is deﬁned by the standard star Vega (m =

0.03). In the purpose of this study we will use the AB magnitude system. The AB magnitude system (Oke [69])is a very useful system because is based on ﬂux

measurements of an monochromatic radiation that is calibrated in ﬂux densities. Its

deﬁnition is given by the equation 5.2.

5 m = − log f − 48.6(erg · s−1 · cm−2 · Hz−1) (5.2) AB 2 10 v

where

λ2 f = f (5.3) v c λ

42 fv it is the ﬂux normalized in frequency units whereas fλ is an expression of wavelength with an estimated error:

√ 2.5dF ∆m = ( )2 (5.4) AB F ln 10

Finally, the ﬂux (F) is related with luminosity (L) and object distance with the

equation :

L F = (5.5) 4πd2

with an error

dF ∆L = (5.6) F ln 10

We use the equation 5.6 in luminosity calculations. For distances of our sam-

ple galaxies we used Virgo-infall-corrected distances from NED (The NASA/IPAC

Extragalactic Database).

At this point it worths to notice that the ﬂux in equation 5.2 is mesured in Jansky

whereas our mesurements were in erg/s/cm2/. So we transformed our ﬂuxes following the relation:

( ) f λ 2 f v = 3.34 × 104 λ (5.7) Jy erg · cm2 · s−1 · −1

Applying all the above in our data we found that due to atmosphere and instrumentation we have a magnitude loss of approximate 10 mAB magnitudes, as we see in table 5.1.

43 Table 5.1: Diﬀerences between mAB magnitudes in sight the atmosphere and through telescope and outside of atmosphere.

Star mAB catalogue mAB measured Feige 34 11.54 20.89

Note: The absolute star ﬂuxes were found in the Bohlin et al. [12] paper.

5.3 Airmass

The light of the stars as it passes through the atmosphere is scattered and absorbed by the atmospheric particles. As thicker is the part of the atmosphere that light passes through, as greater this effect (figure 5.6) is. The result is that when the photons hit the sensor, have diverged in slightly different directions from their original paths, forming a skewed and dimmer image. Airmass (X) measured in zenith position is equal to 1 at airmass scale and gradient increases between the zenith and the horizon to reache the value 2 at the horizon. It is calculated as:

X = secz (5.8)

where z is the angle from zenith. Airmass extinction eﬀects greater the lower altitudes. We have subtract all the data we have used in this project from airmass alteration.

44 Figure 5.6: A schematic representation of the airmass eﬀect.

5.4 Standard Stars

Because every night the weather conditions are different, we need to obtain images of some standard stars. Standard stars are well studied objects (Landolt [63]) with standard fluxes outside the atmosphere in the filter we use. So, in order to establish the transformation between instrumental magnitudes and standard magnitudes we observed many standard stars, which are shown in table 5.2. The standard star images were reduced in the same way as the galaxy images (bias, flat field, bad columns etc).

45 Figure 5.7: The mAB absolute magnitude vs wavelenght (Å) for the Feige34 spectrophotometric standard star.

Figure 5.8: The interface of the Aristarchos Telescope Control Gui environment during an standard star exposure.

46 Table 5.2: Standard star observations by night.

Night Star Airmass Exposure (s)

07/11/15 Feige 110 1.382 240

07/11/15 G191-B2B 1.072 300

07/11/15 G93-48 1.304 360 08/11/15 Feige 110 1.531 330

08/11/15 G93-48 1.229 420

08/11/15 BD+28d4211 1.013 360

08/11/15 G93-48 1.25 420

08/11/15 G93-48 1.315 420 08/11/15 G93-48 1.444 420

08/11/15 G93-48 1.694 600

08/11/15 BD+75d325 1.531 300

08/11/15 G191-B2B 1.086 180 08/11/15 Feige 34 1.127 180

08/11/15 Feige 110 1.812 600

09/11/15 G93-48 1.229 240

09/11/15 Feige 110 1.45 240

09/11/15 BD+75d325 1.515 40 09/11/15 G191-B2B 1.065 30

09/11/15 GRW+70d5824 1.689 720

09/11/15 Feige 34 1.126 600

47 5.5 Photometry

Astronomers observe and calibrate the apparent brightness of an object through the technique of photometry. The general concept is that we measure the number of photons per second, coming from an abject. These observations were done in some specified wavelength range that is defined usually by a filter. Most photometry applications are done in optical and near infrared wavelengths. There are two types of photometry, depending on the object and the study. The aperture and the surface photometry.

5.5.1 Aperture Photometry

The principle behind aperture photometry is to summarize the photon ﬂux that falls within a deﬁned aperture and subtract the contribution of the background emission in the same radius(5.9).

∑ sky I = ijI − n × (5.9) ij pix pixel

where I: total counts in aperture from source, Iij: counts in each pixel in aperture and npix: number in pixels in aperture. The goal of this procedure is to receive only the object’s ﬂux, in order to calculate as we have mentioned already the instrumental magnitude. The aperture radius has to be estimated well because if we apply a large radius we will include light from the background or neighbouring stars. On the other hand, if we apply a small aperture we will loose emitted light. The star boundaries corresponds to a radius given by the Full Width Half Maximum (FWHM). This value is the diameter of the star radius. The sky background is measured from an annulus immediately surrounding the star after the main aperture. The average sky value is substracted from all pixels inside the aperture. In order to decide the best values we must calculate the instrumental magnitude and then construct a plot of the

48 magnitude versus the aperture size. Following this course we will create a curve of growth. The pixel value in which this curve ﬂattening is the optimum aperture radius and sky annulus. From this area will be estimated the sky emission. In our project, we performed aperture photometry for the same stars using 2 pipelines (GAIA and

AstroimageJ) independently and we took almost the same results. Both programs provide to the user an automated calculation of the FWHM as well as the aperture and annulus pixel values.

Figure 5.9: An examble of the aperture photometry. The inner radius measure the star+backround counts, whereas the annulus the sky contribution.

49 5.5.2 Surface Photometry

Surface photometry is a technique to describe the light distribution of extended objects in counts or magnitude. We performed surface photometry using the Gaia pipeline only to the AGN objects of our sample. This was because, we wanted to compare our result with the results of the Theios et al. [83] paper VI. The program reads an image section and gives as output the ﬁtted isophotes parameters, the estimated background , the galaxy counts and the included area in pixels. The results are shown in the table 5.3.

Table 5.3: Our Measurements in Ha+ Continuum. All ﬂuxes and magnitudes are cleaned from [NII] line and galaxy extinction contamination as we described in ref

Name FHa Log(LHa) mAB Error mAB Error FHa

NGC 7469 4.5277−14 40.39 21.52 0.08 2.7082−15

NGC 863 4.0225−14 40.79 21.74 0.08 2.2882−15 MRK 612 9.4732−14 40.54 21.01 0.09 5.2422−15

NGC 1019 9.1535−14 41.06 20.84 0.11 7.0265−15

NGC 37861 8.0985−14 40.14 20.99 0.08 4.6902−15

UGC 12138 1.6348−14 40 22.59 0.08 9.1147−16 IRAS 00160-0719 1.2474−14 39.96 15.73 0.10 9.4272−16

NGC 1194 3.8748−14 40.16 21.67 0.11 3.0596−15

NGC 1241 1.4472−12 41.93 17.73 0.13 1.3005−13

NGC 1320 8.7115−14 40.55 20.85 0.12 8.0567−15

NGC 37862 1.22481−13 40.35 20.49 0.12 1.128−14

−2 −1 Note: The quantity FHa is in erg/cm /s units, the Log(LHa) in ergs

50 CHAPTER VI

Results and Conclusions

In this section we present our photometry results while the conclusions are presented brieﬂy in the next sections.

6.1 Comparison with the bibliography

In order to check our methodology, we compared our photometric results of some galaxies which those that we have in common with Theios et al. [83].

Table 6.1: The Log(LHa) comparison between this paper and Theios et al. [83]

⋄ ∗ Object Log(LHa) Log(LHa)

NGC 7469 40.39 42.27

NGC 1194 40.16 40.25 NGC 1241 41.93 41.78

NGC 1320 40.55 41.34

⋄ ∗ Note: The log(LHa) are the luminosities estimated in this paper and the log(LHa)

−1 are those of Theios et al. [83]. The units of Log(LHa) are erg · s

51 Figure 6.1: log(L) of the currrent work versus log(L) of Theios et al. [83]. The diagonal line represents the equality line between the diﬀerent measurements. We can see that there are some diﬀerences, but there are several reasons for this. The most crucial are the weather conditions, since we faced problematic photometric atmospheric conditions with heavy cirrus clouds during some of our observation sessions.

52 We corrected the observed values given in Table 2 for [N ii] emission by assuming that the 25% of the measured ﬂux belongs to [N ii], based on the work of (Kennicutt

[49]). To convert the observed ﬂuxes into luminosities, we used Virgo-centric infall- corrected distances from NED.

6.2 Photometry between AGN and neighbours

Our observations in the Ha ﬁlter of the area that we have selected to examine for starforming events, are presented below. The labelled galaxy is the AGN neighbour.

The selection of the area to be examined was based on visual criteria using the SDSS broad band images, in order to exclude the emission regions which belong to the galactic spiral structure. In some cases, our objects were not in the SDSS and in such cases we used the 2MASS catalogue (6.5) or other references (6.6 Koulouridis et al.

[57]). Note that by this approach we wish to investigate whether there are extended low-surface brightness Ha emission regions in between the AGN-neighbor pair.

53 Figure 6.2: Left: The Sy2 NGC 3786 and its neighbour NGC 3788. The upper panel shows the SDSS broad-band composite image while the lower panel shows our narrow-band Ha data.

54 Figure 6.3: The pair Sy2 NGC 1320. The upper panel shows the SDSS broad-band composite image while the lower panel shows our narrow-band Ha data.

55 Figure 6.4: The pair Sy2 UGC 12138. The upper panel shows the SDSS broad-band composite image while the lower panel shows our narrow-band Ha data.

56 Figure 6.5: Left: The pair Sy2 NGC 1241. The upper panel shows the SDSS broad- band composite image while the lower panel shows our narrow-band Ha data.

57 Figure 6.6: Left: The pair Sy2 IRAS 00160-0719. The upper panel shows the SDSS broad-band composite image while the lower panel shows our narrow-band Ha data.

58 Figure 6.7: Left: The pair Sy2 MRK 612. The upper panel shows the SDSS broad- band composite image while the lower panel shows our narrow-band Ha data.

59 Figure 6.8: Left: The pair Sy2 NGC 7682. The upper panel shows the SDSS broad- band composite image while the lower panel shows our narrow-band Ha data. These object were selected as control sample.

60 Figure 6.9: Left: The pair Sy1 NGC 863. The upper panel shows the SDSS broad- band composite image while the lower panel shows our narrow-band Ha data.

61 Figure 6.10: Left: The pair Sy1 NGC 1019. The upper panel shows the SDSS broad- band composite image while the lower panel shows our narrow-band Ha data.

62 Figure 6.11: The pair Sy1 NGC 1194. The upper panel shows the SDSS broad-band composite image while the lower panel shows our narrow-band Ha data.

63 The photometric results of all the above galaxies are presented below in three plots. As we can see, the ﬂux diﬀerences between the area that we are interested in, and the background are consistent within the uncertainties. There is no indication of a low-surface brightness Ha emission in the region between the AGN and their neighbours.

We also divided our sample between Sy1 and Sy2, in order to examine if we could find a difference between the two types. This was not possibly because of the small sample of galaxies that we managed to observe. It is possible that a bigger sample with longer exposures could reveal some indications of differences between them.

Figure 6.12: Ha F luxbetween versus Ha F luxbackground for the Seyfert 1 case.

64 Figure 6.13: Ha F luxbetween versus Ha F luxbackground luminosities for the Seyfert 2 cases.

Figure 6.14: Ha F luxbetween versus Ha F luxbackground of all our objects. As we can see, the ﬂux diﬀerences between the area that we are interested in, and the background are within the 1σ uncertainty.

65 6.2.1 Individual “Blob” Photometry

The fact that we could not detect any signiﬁcant extended Ha emission regions does not mean that there is no other possibility for emission in the area that we are interested in. There are some visual indications that there are small Ha emission regions that could have been formed due to interactions between the AGN and its neighbour. In order to determine if such emission regions belong to the AGN- neighbour galaxy system we checked if they have the same redshift, using the SDSS

KD-tree method. In IRAS 00160-0716 we have found that there is an Ha emission

“blob” with a redshift identical to that of the galaxy.

6.2.1.1 IRAS 00160-0716

Figure 6.15: IRAS 00160-0719 and its neighbour objects based on the SIMBAD catalogue.

66 Figure 6.16: The “Blob” suspected of being part of the AGN-neighbour system in circled. The estimated redshift (with KD tree method) placed this blob quite close to Sy 2 galaxy. This is circustantial evidence that IRAS 00160-0719 could be a hidden Sy 1 galaxy.

Figure 6.17: The 3D surface plot of this area showing the counts per pixel.

6.2.1.2 NGC 7682

We have chose the NGC 7682 and its neighbour because there is an intense ac- tivityin the intermediate region, which is also obvious in the SDSS broad band ﬁlter.

Also the neighbour is tidally disturbed and this is the reason for which we used this as our control sample.

67 Figure 6.18: This “blob” is estimated to have z = 0.160 0.080. Notice that the estimated by this method AGN redshift deviates signiﬁcantly from its measured value; however we believe that negligible relative z diﬀerences (even if the absolute values are wrong) could indicate a physical association of the “blob” with the AGN-neighbour system.

Figure 6.19: The selected area around the “blob” as a 3D plot which the x and y axis represent the dimension of our selection and z-axis the count values.

68 Figure 6.20: The second “blob” which we spotted. In this case there is no available estimation of its z.

Figure 6.21: “Blob” 2 area in a 3D plot.

69 Figure 6.22: This “blob” is labelled as “blob” 3. Although, there is Ha emission, comparing the estimated “blob” and galaxy redshifts show that it is probably a background object and thus not associated with the AGN-neighbour system.

Figure 6.23: “Blob” 3 area in a 3D plot.

70 Figure 6.24: “Blob” 4. Again, based in the SDSS KD-tree method this blob does not appear to belong to the system.

Figure 6.25: “Blob 4” area in a 3D plot.

71 Figure 6.26: “Blob” 5. Based in the SDSS KD-tree method this blob does not appear to belong to the system.

Figure 6.27: Blob 5 area in a 3D plot.

72 6.2.2 NGC 7469

In this Seyfert 1 galaxy we have observed that the Ha emission is more intense in the AGN host-galaxy spirals which are towards the neighbour. This indicates that these regions have a higher star formation rate with respect to those in the anti-diametric spiral.

Figure 6.28: NGC 7469. The intense starformation in the arm of NGC 7469 towards the neighbour is signiﬁcantly higher than that of the opposite side.

73 6.2.2.1 Conclusions

The detailed data analysis of this pilot study has shown that there are some indications for the existence of Ha emission regions in between Seyfert 2 galaxies and their neighbours, with respect to that of Sy1’s. In fact we have found that although there are no extended Ha emission regions, some Ha “blobs” and small Ha emission areas do exist in some cases. Such a fact could be an indication for an evolutionary component of the AGN phenomenology. If indeed there are more star forming regions in between Sy2 AGN and their closest neighbour, with respect to Sy1, then according to theoretical models it is possible that some Seyfert 2 galaxies are in fact Seyfert 1 with an induced dust and star forming cocoon around their nuclei. However, in order to arrive to such an important result, we need to verify the current suggestions with a much larger statistical sample and the relevant observations.

74 APPENDICES

75 APPENDIX A

Calculations

Calculation of Night Constants

The probability to take a measurement of a quantity y, for a given x, when the theoretical value is f(x) and the total error of the experimental measurement is σ is:

1 1 y − f(x) P (x) = √ exp[− ( )2] (A.1) σ 2π 2 σ

In our case this quantity is the instrumental magnitude, minst, of each standard star that we have observed. We have obtained the true apparent magnitude of our standard stars from Landolt [63]. The total error σm of the measurement is estimated as follows, since the magnitude of a star is proportional to the intensity:

minst = −2.5logA + z (A.2)

where minst is the observed instrumental magnitude and A =Counts/exposure time (this quantity has already been corrected for sky emission) and z is the zero-point constant of the night. We calculate the magnitude error as a function of the Signal

76 to Noise Ratio (S/N). The signal to noise ratio is a technical value which represent the quality of the signal. In our case the S/N is given by the ratio of the light signal to the sum of noise signals (photon noise, read noise, sky noise, etc). The signal to noise ratio is given by the equation A.3.

S I − N I = √ s pix sky (A.3) − σ2 N Is NpixIsky 2 sky g + Npixσsky + Npix p where Is is the star counts within an aperture, g is the gain (electrons/counts ratio),

Npix is the number of pixel within the aperture, Isky is the background sky ﬂux

2 (counts/pixel), p is the pixels of the sky aperture area and σsky is the root mean square (rms) value in the sky aperture. The referenced quantities are given by iraf pipeline with the usage of STSDAS package - imexamine task.

Finally, the error in magnitude is:

2.5 1 σ = (A.4) m ln10 S/N

Next, with the intention to calculate the night constants, ie., the extinction co- eﬃcient k and zero-point z, we will use the fact that in absolute photometry these quantities are related via the equation:

minst = m + z + k × X (A.5)

where minst the instrumental magnitude value provided by eq.(A.2), m is the true apparent magnitude in the Ha ﬁlter, provided in Landolt [63] and X the airmass of the observed star. Using the so-called χ2-squared minimization procedure it is possible to calculate the night constants (zero-point constant z and extinction coeﬃcient k) by comparing the observed instrumental magnitude minst with the theoretical one provided by

77 eq.(A.5). This statistical method is the most common procedure to ﬁnd the parameters for which our observational data are ﬁtted well enough by some theoretical

predictions: ∑ m − (m + z + k × X) x2 = ( inst )2 (A.6) σ

We calculate the free parameters k and z using the following code in Fortran 95.

program main implicit none integer:: i, j, t, io, jo, k, zp, N=9 real:: chiz, chimin, kmin, zpmin,ak,azp,aaa

REAL, DIMENSION(9) :: m ! uncalibrated magnitudes

REAL, DIMENSION(9) :: a ! air mass constants

REAL, DIMENSION(9) :: mobs ! catalog magnitudes

REAL, DIMENSION(9) :: sigma ! catalog magnitudes REAL, DIMENSION(9) :: mk ! calibrated magnitudes

!read the data open(1, file=’magn.txt’) open(2, file=’air.txt’) open(3, file=’mobs.txt’) open(4, file=’sigma.txt’)

aaa=0 do i=1,N read(1,*) m(i) read(2,*) a(i) read(3,*) mobs(i)

78 read(4,*) sigma(i) c print*,mobs(i)-m(i) aaa=aaa+mobs(i)-m(i) end do print*,’Simple mean mzp=’,aaa/N jo=0 chimin=1e10

do zp=1750,2550 jo=jo+1 chiz=0 azp=(ﬂoat(zp)/100.)

do i=1,N mk(i)=m(i)+azp chiz=chiz+( (mobs(i)-mk(i)) )**2 end do if(chiz.lt.chimin) then chimin=chiz zpmin=azp end if write(89,*) jo,azp,chiz end do

print *,’1param. ﬁt:’,chimin/(N-1),’ mzp=’,zpmin

chimin=1e10 io=0

79 do k=0,800 ak=ﬂoat(k-400)/100. io=io+1 jo=0 do zp=1750,2550 jo=jo+1 chiz=0 azp=(ﬂoat(zp)/100.)

do i=1,N mk(i)=m(i)-ak*a(i)+azp c chiz=chiz+( (mobs(i)-mk(i)) /(150000.*sigma(i)) )**2 chiz=chiz+( (mobs(i)-mk(i)) )**2 end do if(chiz.lt.chimin) then chimin=chiz kmin=ak zpmin=azp end if write(88,*) io,jo,ak,azp,chiz end do end do print *,’2param. ﬁt:’,chimin/(N-2),’ k=’,kmin,’ mzp=’,zpmin return end

80 Figure A.1: The solution space resulting from the χ2-minimization procedure where both the k constant and zero-point z ﬁtted parameters are free. These results correspond to the last night of observations(09/11/15). Notice that the k and z parameters are quite degenerate, although the accuracy of z is quite satisfying.

We have calculated both k and z, allowing both to vary within a range of plausible values. The results are shown in Figure A.1 where the contours correspond to the 1,

2 and 3σ uncertainty range. Since the solution is degenerate and in order to provide speciﬁc uncertainties for each free parameter, we keep one parameter constant to its best value and allow the other to be ﬁtted by the data. This procedure is called marginalization of the one parameter over the other. Note, that the 1σ uncertainty

2 − 2 which corresponds to the two free parameter case is x xmin = 2.3, which is the value that we also use for the case where we marginalize each parameter over the other.

The ﬁnal results for the night that we use as an example here (09/11/15), are

81 k = −0.11 0.35 and z = 19.61 0.48. Note that the values of k and z found for the ﬁrst problematic night were not used since they where extremely degenerate and were replaced by the corresponding values of the next night.

82 APPENDIX B

Filter Constant

Calculation of Filter Constant

In this section we present the procedure used to calculate the so called ”counts to

energy” conversion factor Sλ, which is necessary for the calculation of the absolute

ﬂux Fλ of a source that has been observed. This factor measures the eﬃciency of the telescope and the camera (as a system) and it can be calculated using measurements

from standard stars. The equation for the absolute ﬂux is:

0.4kλX SλCλ10 Fλ = (B.1) Tλ

where Cλ is the count rate, kλ is the extinction coeﬃcient, X is the airmass and Tλ is the transmission peak in the band-width ∆λ. Considering n standard stars and the fact that the ﬂux of the standard star can be considered constant (F0) in the center

of the band, one can calculate the Sλ for each standard star, as follows:

F0Wλ Sλ = 0.4k X (B.2) Clambda10 λ

83 where Wλ is the equivalent width of the ﬁlter. The mean of these values for Sλ

can be used in equation (B.1) to convert the measured count rate Cλ to absolute ﬂux

Fλ.

The uncertainty in Sλ and Fλ can be easily calculated if we know the uncertainty in k and C.

The total uncertainty in Sλ is:

∂S ∂S ∆S2 = ( )2dk2 + ( )2dC2 (B.3) ∂K ∂C

hence: √ ∆S2 = S (0.4x/ln10)2dk2 + (1/C)dC2 (B.4)

Similarly, the total uncertainty in Fλ is

∂F ∂F ∂F ∆F 2 = ( )2dS2 + ( )2 + dk2 (B.5) ∂S ∂C partialk

hence: F ∆F 2 = ( )2[C2dS2 + S2dC2 + (0.4SCx/ln10)2dk2] (B.6) SC

84 BIBLIOGRAPHY

85 BIBLIOGRAPHY

[1] R. Antonucci and J. Miller. Diﬀuse pulmonary hemorrhage: a review and clas- siﬁcation. Astrophysical Journal, 297:621–632, 1985. doi: 10.1155/2012/782030.

[2] Robert Antonucci. Uniﬁed models for active galactic nuclei and quasars. Annual review of astronomy and astrophysics, 31:473–521, 1993. doi: 10.1146/annurev. aa.31.090193.002353.

[3] H. Awaki, H. Kunieda, Y. Tawara, and K. Koyama. Discovery of strong absorption in the x-ray spectrum of seyfert 2 galaxy ngc 4507. Astronomical Society of Japan, 336:L37–L42, 1991.

[4] W. Baade and R. Minkowski. On the identiﬁcation of radio sources. Astrophysical Journal, 119:215, 1954. doi: 10.1086/145813.

[5] J. A. Baldwin, M. M. Phillips, and R. Terlevich. Classiﬁcation parameters for the emission-line spectra of extragalactic objects. Astronomical Society of the Paciﬁc, Publications, 93:5–19, 1981. doi: 10.1086/130766.

[6] D. Ballantyne, Y. Shi, G. H. Rieke, J. L. Donley, C. Papovich, and J. R. Rigby. Does the agn uniﬁed model evolve with redshift? using the x-ray background to predict the mid-infrared emission of agns. Astrophysical Journal, 653(2):1070, 2006. doi: 10.1086/508801.

[7] Peter D. Barthel. Is every quasar beamed? Astrophysical Journal, 336:606–611, 1989. doi: 10.1086/167038.

[8] Volker Beckmann and Chris Shrader. Active Galactic Nuclei. WILEY-VCH Verlag GmbH and Co. KGaA, Germany, 2012.

[9] Luciana Bianchi, David A. Thilker, Denis Burgarella, Peter G. Friedman, Charles G. Hoopes, Samuel Boissier, Armando Gil de Paz, Tom A. Barlow, Yong- Ik Byun, Jose Donas, Karl Forster, Timothy M. Heckman, Patrick N. Jelinsky, Young-Wook Lee, Barry F. Madore, Roger F. Malina, D. Christopher Martin, Bruno Milliard, Patrick Morrissey, Susan G. Neﬀ, R. Michael Rich, David Schimi- novich, Oswald H.W. Siegmund, Todd Small, Alex S. Szalay, Barry Y. Welsh, and Ted K. Wyder. Recent star formation in nearby galaxies from galaxy evolution explorer imaging: M101 and m51. The Astrophysical Journal Letters, 619 (1):L71, 2005.

86 [10] R. D. Blandford and A. Konigl. Relativistic jets as compact radio sources. As- trophysical Journal, 232:34–48, 1979. doi: 10.1086/157262.

[11] Bia Boccardi, Thomas Krichbaum, Uwe Bach, Eduardo Ros, and J. Anton Zen- sus. High resolution mm-vlbi imaging of cygnus a. 12th European VLBI Network Symposium and Users Meeting, 2014.

[12] R. C. Bohlin, M. E. Dickinson, and D. Calzetti. Spectrophotometric standards from the far-ultraviolet to the near-infrared: Stis and nicmos ﬂuxes. Astrophysical Journal Supplement, 27:21, 1974. doi: 10.1086/190287.

[13] N. Bohr. Rydberg’s discovery of the spectral laws. Kalckar J. Collected works, 10:373–379, 1985.

[14] Catherine L. Buchanan, Gallimore Jack, Axon David, Baum Steﬁ, Elitzur Moshe, Elvis Martin, O Dea Christopher, and Robinson Andrew. Infrared seds of seyfert galaxies: Starbursts and the nature of the obscuring medium. Spitzer Proposal ID 3269, 50, 2004.

[15] Howard A. Bushouse and Michael W. Werne. Near-infrared imaging of interacting galaxies. Astronomical Journal, 359:72–85, 1990. doi: 10.1086/169035.

[16] Neven Caplar, Simon J. Lilly, and Benny Trakhtenbrot. Agn evolution from a galaxy evolution viewpoint. The Astrophysical Journal, 811(2):148, 2015.

[17] Li Cheng, Kauﬀmann Guinevere, Heckman Timothy M., White Simon D. M., and Jing Y. P. Interactions star formation and agn activity. Monthly Notices of the Royal Astronomical Society, 62:1915–1922, 2008. doi: 10.1111/j.1365-2966. 2008.13023.x.

[18] Li Cheng, Guinevere Kauﬀmann Timothy, M. Heckman, Simon D. M. White, and Y. P. Jing. Interactions, star formation and agn activity. Monthly Notices of the Royal Astronomical Society, 385:1915–1922, 2008. doi: 10.1111/j.1365-2966. 2008.13023.x.

[19] Heber Doust Curtis. Descriptions of 762 nebulae and clusters photographed with the crossley reﬂector. Publications of the Lick Observatory, 13, 1918.

[20] O. Dahari. The nuclear activity of interacting galaxies. Astrophysical Journal Supplement Series, 57:643–664, 1985. doi: 10.1086/191021.

[21] R. J. Davis and T. W. B. Muxlow ND R. G. Conway. Radio emission from the jet and lobe of 3c273. NATURE, 318(343–345), 1985. doi: 10.1038/318343a0.

[22] C. L. Dobbs, C. Theis, J. E. Pringle, and M. R. Bate. Simulations of the grand design galaxy m51: a case study for analysing tidally induced spiral structure. Monthly Notices of the Royal Astronomical Society, 403(2):625–645, 2010. doi: 10.1111/j.1365-2966.2009.16161.x.

87 [23] Hacyan Dultzin, Y. Krongold, I. Fuentes-Guridi, and P. Marziani. The close environment of seyfert galaxies and its implication for uniﬁcation models. As- trophysical Journal, 513:L111–L114, 1999. doi: 10.1086/311925.

[24] A. S. Eddington. The conditions of emission of forbidden lines. Monthly Notices of the Royal Astronomical Society, 88:134–138, 1927.

[25] Moshe Elitzur. The obscuring torus in agn. New Astronomy Reviews, 50:728–731, 2006. doi: 10.1016/j.newar.2006.06.027.

[26] Sara L. Ellison, David R. Patton, Luc Simard, and Alan W. McConnachie. Galaxy pairs in the sloan digital sky survey. i. star formation, active galactic nucleus fraction, and the luminosity/mass-metallicity relation. The Astronomi- cal Journal, 135(5):1877, 2008. doi: 10.1088/0004-6256/135/5/1877.

[27] B. G. Elmegreen and M. Thomasson. Grand design and ﬂocculent spiral structure in computer simulations with star formation and gas heating. Astronomy and Astrophysics, 272(1):37, 1993.

[28] M. Elvis and F. Nicastro. Ubiquitous variability of x-ray-absorbing column densities in seyfert 2 galaxies. Astrophysical Journal, 571(1):234, 2002.

[29] A. Elyiv, N. Clerc, M. Plionis, J. Surdej, M. Pierre, S. Basilakos, L. Chiappetti, P. Gandhi, E. Gosset, O. Melnyk, and F. Pacaud. Angular correlation functions of x-ray point-like sources in the full exposure xmm-lss ﬁeld. Astronomy and Astrophysics, 537:14, 2012. doi: 10.1051/0004-6361/201117983.

[30] J. Daniel Mortlock et al. A luminous quasar at a redshift of z = 7.085. Nature, 474:619, 2011. doi: 10.1038/nature10159.

[31] R. Singh et al. The nature of liner galaxies: Ubiquitous hot old stars and rare accreting black holes. Astronomy and Astrophysics, 558(43), 2013. doi: 10.1051/ 0004-6361/201322062.

[32] B. L. Fanaroﬀ and J. M. Riley. The morphology of extragalactic radio sources of high and low luminosity. Monthly Notices of the Royal Astronomical Society, 167(31–36), 1985. doi: 10.1093/mnras/167.1.31p.

[33] E. A. Fath. The spectra of some spiral nebulae and globular star clusters. Pub- lications of the Astronomical Society of the Paciﬁc, 21(126):138–143, 1909. doi: 10.1086/121907.

[34] H. B. French and J. S. Miller. The spectrum of the variable nucleus and surrounding nebulosity of 3c 120. Astronomical Society of the Paciﬁc, Publications, 92:31–36, 1980.

[35] G. Ghisellini and T. Sbarrato. Dark bubbles around high-redshift radio-loud agn. Monthly Notices Letters, 2016. doi: 10.1093/mnrasl/slw089.

88 [36] Robert W. Goodrich. Spectropolarimetry of narrow-line seyfert 1 galaxies. As- trophysical Journal, 342:224–234, 1989. doi: 10.1086/167586. [37] G. Hasinger. Absorption properties and evolution of active galactic nuclei. Astronomy and Astrophysics, 490(1):905–922, 2008. doi: 10.1051/0004-6361: 200809839. [38] T. M. Heckman. An optical and radio survay of the nuclei of bright galaxies. Astronomy and Astrophysics, 87:14–151, 1980. [39] Ryan Hickox, Christine Jones, Forman William R, Murray Stephen S, Kochanek Christopher S, Eisenstein Daniel, Jannuzi Buell T, Dey Arjun, Brown Michael J, Stern Daniel, Eisenhardt Peter R, Gorjian Varoujan, Brod- win Mark, Narayan Ramesh, Cool Richard J, Kenter Almus, Caldwell Nelson, and Anderson Michael E. Host galaxies, clustering, eddington ratios, and evolution of radio, x-ray, and infrared-selected agns. The Astrophysical Journal, 696: 891–919, 2009. doi: 10.1088/0004-637X/696/1/891. [40] D. Tran Hien. The nature of seyfert 2 galaxies with obscured broad-line regions. ii. individual objects. Astrophysical Journal, 440:578, 1995. doi: 10.1086/175297. [41] J. R. Houck, D. P. Schneider, G. E. Danielson, G. Neugebauer, B. T. Soifer, C. A. Beichman, and C. J. Lonsdale. Unidentified iras sources - ultrahigh-luminosity galaxies. Astrophysical Journal, 290(2):L5–L8, 1985. doi: 10.1086/184431. [42] Edwin Hubble. No. 324. extra-galactic nebulae. Contributions from the Mount Wilson Observatory, 1926. [43] J. B. Hutchings, D. Crampton, B. Campbell, D. Duncan, and B. Glendenning. An atlas of qso luminosity profiles. Astrophysical Journal Supplement Series, 55: 319–366, 1984. doi: 10.1086/190958. [44] Y. Ideue, Y.Taniguchi, T. Nagao, Y. Shioya, M. Kajisawa, J. R. Trump, D. Ver- gani, A. Iovino, A. M. Koekemoer, O. Le Fèvre, O. Ilbert, and N. Z. Scoville. The role of galaxy interaction in environmental dependence of the star formation activity at z 1.2. The Astrophysical Journal, 747(1):42, 2012. [45] Karl G. Jansky. Radio waves from outside the solar system. Nature, 132:66, 1933. doi: 10.1038/132066a0. [46] G. Kauffmann, T. Heckman, S. White, S. Charlot, C. Tremonti, E. Peng, M. Seib- ert, J. Brinkmann, R. Nichol, SubbaRaoM., and D.York. The dependence of star formation history and internal structure on stellar mass for 105 low-redshift galaxies. Monthly Notice of the Royal Astronomical Society, 341(1):54–69, 2003. doi: 10.1046/j.1365-8711.2003.06292.x. [47] W. C. Keel, R. C. Jr. Kennicutt, E. Hummel, and van der J. M. Hulst. The effects of interactions on spiral galaxies. i - nuclear activity and star formation. Astronomical Journal, 90:708–730, 1985. doi: 10.1086/113779.

89 [48] K. Kellermann, R. Sramek, D. Shaﬀer, R. Green, and M. Schmidt. Radio emission from optically selected quasars. Quasars; Proceedings of the IAU Sympo- sium, Bangalore, India, Dec. 2-6, pages 95–100, 1985. doi: 10.1038/nature10159.

[49] R. C. Kennicutt. The rate of star formation in normal disk galaxies. The Astro- physical Journal, 272:54–67, 1983. doi: 10.1086/161261.

[50] Robert C. Kennicutt. Star formation in galaxies along the hubble sequence. Annual Review of Astronomy and Astrophysics, 36:189–232, 1998. doi: 10.1146/ annurev.astro.36.1.189.

[51] L. Kewley and A.Dopita. Using strong lines to estimate abundances in extragalactic h ii regions and starburst galaxies. The Astrophysical Journal Supplement Series, 142:35–52, 2002. doi: 10.1086/341326.

[52] Emil T. Khabiboulline, Charles L. Steinhardt, John D. Silverman, Sara L. Elli- son, J. Trevor Mendel, and David R. Patton. Changing ionization conditions in sdss galaxies with active galactic nuclei as a function of environment from pairs to clusters. Astrophysical Journal, 795(1):62, 2014.

[53] E. Y. Khachikian and D. W. Weedman. An atlas of seyfert galaxies. Astrophysical Journal, 192:581–589, 1974.

[54] Eh. Khachikian and Weedman D. A spectroscopic study of luminous galactic nuclei. Astroﬁzika, 58:389 – 406, 1971.

[55] E. Koulouridis. The dichotomy of seyfert 2 galaxies: intrinsic diﬀerences and evolution. Astronomy and Astrophysics, 570(9):14, 2014. doi: http://dx.doi.org/ 10.1051/0004-6361/201424622.

[56] E. Koulouridis, M. Plionis, V. Chavushyan, V. Dultzin, Y. Krongold, and C. Goudis. Local and large-scale environment of seyfert galaxies. Astrophys- ical Journal, 639:37–45, 2006. doi: 10.1086/498421.

[57] E. Koulouridis, M. Plionis, V. Chavushyan, V. Dultzin, Y. Krongold, I. Geor- gantopoulos, and Leon Tavares J. Activity of the seyfert galaxy neighbours. Astronomy and Astrophysics, 552(A135), 2013. doi: http://dx.doi.org/10.1051/ 0004-6361/201219606.

[58] Elias Koulouridis, Vahram Chavushyan, Manolis Plionis, Yair Krongold, and Deborah Dultzin-Hacyan. A three-dimensional study of the local environment of bright iras galaxies: The active galactic nucleus-starburst connection. The Astrophysical Journal, 1651(1):93–100, 2006.

[59] J. H. Krolik. Active Galactic Nuclei : From the Central Black Hole to the Galactic Environment. Princeton University Press, USA, 1999.

90 [60] Julian H. Krolik and Begelman Mitchell C. Molecular tori in seyfert galaxies - feeding the monster and hiding it. Astrophysical Journal, 329:702–711, 1988. doi: 10.1086/166414.

[61] Y. Krongold, Hacyan Dultzin, and P. Marziani. The circumgalactic environment of bright iras galaxies. Astrophysical Journal, 572:169–177, 2002. doi: 10.1086/ 340299.

[62] Stephanie M. LaMassa, Sabrina Cales, Edward C. Moran, Adam D. Myers, Gor- don T. Richards, Michael Eracleous, Timothy M. Heckman, Luigi Gallo, and C. Megan Urry. The discovery of the ﬁrst changing look quasar: New insights into the physics and phenomenology of active galactic nucleus. Astrophysical Journal, 800(2):144, 2014.

[63] A. Landolt. Ubvri photometric standard stars in the magnitude range 11.5-16.0 around the celestial equator. Astronomical Journal, 104:340–371, 1992.

[64] N. A. Levenson, J. T. Radomski, C. Packham, R. E. Mason, J. J. Schaefer, and C. M. Telesco. Isotropic mid-infrared emission from the central 100 pc of active galaxies. The Astrophysical Journal, 703:390–398, 2009. doi: 10.1088/ 0004-637X/703/1/390.

[65] Bell D. Lynden and A. J. Kalnajs. On the generating mechanism of spiral structure. Monthly Notices of the Royal Astronomical Society, 157:1, 1972. doi: 10.1093/mnras/157.1.1.

[66] D. Lynden-Bell. Galactic nuclei as collapsed old quasars. Nature, 223:690–694, 1969. doi: 10.1038/223690a0.

[67] M. A. Malkan and W. L. W. Sargent. The ultraviolet excess of seyfert 1 galaxies and quasars. ApJ, 254:22–37, 1982. doi: 10.1086/159701.

[68] Andrea Merloni and Sebastian Heinz. Evolution of Active Galactic Nuclei. Springer Netherlands, Dordrecht, 2012.

[69] J. B. Oke. Absolute spectral energy distributions for white dwarfs. Astrophysical Journal Supplement, 27:21, 1974. doi: 10.1086/190287.

[70] D. Osterbrock. Seyfert galaxies with weak broad h alpha emission lines. Astro- physical Journal, 249:462–470, 1981. doi: 10.1086/159306.

[71] D. Osterbrock. Astrophysics of gaseous nebulae and active galactic nuclei. Re- search supported by the University of California, John Simon Guggenheim Memo- rial Foundation, University of Minnesota, et al. Mill Valley, CA, University Science Books, page 422, 1989.

[72] B. Peterson. An Introduction to Active Galactic Nuclei. Cambridge University Press, New York, 1997.

91 [73] E. E. Salpeter. Accretion of interstellar matter by massive objects. Astrophysical Journal, 140:769–800, 1964. doi: 10.1086/147973.

[74] B. Sanders, B. T. Soifer, J. H. Elias, B. F. Madore, K. Matthews, G. Neugebauer, and N. Z. Scoville. Ultraluminous infrared galaxies and the origin of quasars. The Astrophysical Journal, 325:74–91, 1998. doi: 10.1086/165983.

[75] M. Schmidt. 3c 273 : A star-like object with large red-shift. Nature, 197:1040, 1963. doi: 10.1038/1971040a0.

[76] M. Schmidt and R. F. Green. Quasar evolution derived from the palomar bright quasar survey and other complete quasar surveys. Astrophysical Journal, 269: 352–374, 1983. doi: 10.1086/161048.

[77] Carl K. Seyfert. Nuclear emission in spiral nebulae. Astrophysical Journal, 97: 28, 1943. doi: 10.1086/144488.

[78] G. A. Shields and L. Searle. The composition gradient across m101. Astrophysical Journal, 222:821–832, 1978. doi: 10.1086/156201.

[79] Isaac Shlosman. Mass-Transfer Induced Activity in Galaxies. Cambridge Uni- versity Press, UK, 2003.

[80] Ralf Siebenmorgen, Frank Heymann, and Andreas Efstathiou. Self-consistent 2- phase agn torus models. Astronomy and Astrophysics, 583(A120):18, 2015. doi: http://dx.doi.org10.1051/0004-6361/201526034.

[81] V. M. Slipher. Nebulae. Proceedings of the American Philosophical Society, 56: 403–409, 1917.

[82] B. T. Soifer and G. Neugebauer. The properties of infrared galaxies in the local universe. Astrophysical Journal, 101:354–361, 1991.

[83] Rachel Theios, A. Matthew Malkan, and Ross Nathanie. Ha imaging of nearby seyfert host galaxies. The Astrophysical Journal, 822(1):45, 2016.

[84] E. Treister, N. Carolin Cardamone, Kevin Schawinski, C. Megan Urry, Eric Gawiser, Shanil Virani, Paulina Lira, Jeyhan Kartaltepe, Maaike Damen, Ed- ward N. Taylor, Emeric Le Floc’h, Stephen Justham, and Anton M. Koekemoer. Heavily obscured agn in star-forming galaxies at z = 2. Astrophysical Journal, 706(1):535, 2009.

[85] C. Megan Urry and Paolo Padovani. Uniﬁed schemes for radio-loud active galactic nuclei. Publications of the Astronomical Society of the Paciﬁc, 107:803, 1995. doi: 10.1086/133630.

[86] S. Veilleux and D. Osterbrock. Spectral classiﬁcation of emission-line galaxies. Astrophysical Journal Supplement Series, 63:295–310, 1987. doi: 10.1086/191166.

92 [87] R. C. Vermeulen, P. M. Ogle, H. D. Tran, I. W. A. Browne, M. H. Cohen, A. C. S. Readhead, G. B. Taylor, and R. W. Goodrich. When is bl lac not a bl lac? The Astrophysical Journal Letters, 452(1):L5, 1995.

[88] Beatriz Villarroel and Andreas J. Korn. The diﬀerent neighbours around type- 1 and type-2 active galactic nuclei. Nature Physics, 10:417–420, 2013. doi: 10.1038/nphys2951.

[89] Beatriz Villarroel and Andreas J. Korn. The diﬀerent neighbours around type- 1 and type-2 active galactic nuclei. Nature Physics, 10:417–420, 2014. doi: 10.1038/nphys2951.

[90] Thuan Trinh Xuan, Balkowski Chantal, and Tran Thanh Van J. Physics of nearby galaxies: Nature or nurture? Atlantica Séguier Frontieres, France, 1992.

[91] R. Yan and M. Blanton. Are liners agn? American Astronomical Society, 2012.

[92] Y. B. Zeldovich and Novikov. Soviet Physics Doklady, 9:195, 1964.