Isolated Elliptical Galaxies Fatma Mohamed Reda Mahmoud Mohamed (M.Sc.) A thesis presented in fulfilment of the requirements of Doctor of Philosophy. Faculty of Information and Communication Technology Swinburne University of Technology 2007 Abstract This thesis presents a detailed study of a well defined sample of isolated early-type galaxies. We define a sample of 36 nearby isolated early-type galaxies using a strict isolation criteria. New wide-field optical imaging of 20 isolated galaxies confirms their early-type morphology and relative isolation. We find that the isolated galaxies reveal a colour-magnitude relation similar to cluster ellipticals, which suggests that they formed at a similar epoch to cluster galaxies, such that the bulk of their stars are very old. However, several galaxies of our sample reveal evidence for dust lanes, plumes, shells, boxy and disk isophotes. Thus at least some isolated galaxies have experienced a recent merger/accretion event which may have also induced a small burst of star formation. Using new long-slit spectra of 12 galaxies we found that, isolated galaxies follow similar scaling relations between central stellar population parameters, such as age, metallicity [Z/H] and α-element abundance [E/Fe], with galaxy velocity dispersion to their counterparts in high density environments. However, isolated galaxies tend to have slightly younger ages, higher metallicities and lower abundance ratios. Such properties imply an extended star formation history for galaxies in lower density environments. We measure age gradients that anticorrelate with the central galaxy age. Thus as a young starburst evolves, the age gradient flattens from positive to almost zero. Metallicity gradients range from near zero to strongly negative. For our high mass galaxies, metallicity gradients are shallower with increasing galaxy mass. Such behaviour is expected in the remnants of multiple mergers. The metallicity gradients are also found to be correlated with the central age and metallicity, as well as to the age gradients. We generally find flat [E/Fe] gradients. We also examine the Fundamental Plane in both traditional Re, µe and σ space and κ-space. Most isolated galaxies follow the same Fundamental Plane tilt and scatter for galaxies in high density environments. However, a few galaxies notably deviate from the plane in the sense of having smaller M/L ratios. This can be understood in terms of their younger stellar populations, which are presumably induced by a gaseous merger. In conclusion, our results are compatible with an extended merger/accretion history for most isolated elliptical galaxies. However, for those galaxies which show no fine structures nor any young stellar populations, an early formation epoch followed by passive evolution is more probable. i ii Acknowledgements First and foremost I would like to thank my supervisor Duncan Forbes for giving me the opportunity to join his team at Swinburne University. I appreciate his incredible support starting from the stage of writing my first paper in astronomy and ending with this thesis. I greatly appreciate his unstinting sharing of expertise, knowledge and his suggestions, input, calm encouragement and remarkable patience. Thanks Duncan. I am also very grateful to my co-advisor Robert Proctor for being very supportive and encouraging during my work with the stellar populations part of this study. He did not only provide me with his programming codes which were essential for this study, but also his advise and discussions were of great impact on my understanding of the details of this area of astrophysics. Thanks also to his insightful comments on the earlier drafts, unending patience and encouragements. Thanks Rob. My deep thanks to George Hau whose discussions and consultation, during his visits to Swinburne or via email, significantly assisted me to accomplish this thesis. Thanks also to George for supplying me with the reduced 2-D spectra. In particular I would like to thank Matthew Bailes for approving me as a visitor to the Center for more than two years and then as a formal student at Swinburne. I am really grateful to him for supplying me with all facilities that I needed. I would also like to express my thanks to Lee Spitler, Max Spolaor, Trevor Mendel, Virginia Kilborn, Sarah Brough and Michael Pierce for the useful discussions and brainstorming during our regular meetings as well as other casual talks and conversations. I greatly appreciate and thank Michael Beasley, Ewan O’Sullivan and Søren Larsen for participating with various aspects throughout this study. I am also grateful to the supporting team at the Anglo Australian Observatory. I would like also to thank Paul Goudfrooij and Vera Kozhurina-Platais for supplying us with the reduced images observed with the ESO/MPG telescope and Ale Terlevich for his help with the observations. I acknowledge and thank the generous support from the sponsors of my PhD scholarship. My living expenses for the first two years and travel fares were provided by The Egyptian Ministry of Higher Education. The Centre for Astrophysics & Supercomputing, the Faculty iii of Information and Communication Technology and Swinburne University provided a full scholarship for the last year. I would also like to acknowledge a travel fund provided by the IAU Travel Grant Program. Next, I want to thank all my good friends whom I met in Melbourne: Yasmine, Roza, Ika, Aisha, Dayana, Nora, Suzanne and Shereen. We shared lots of fun, happiness, laughter as well as tears. Their love and care was very supportive during my studying years, although, that make it more painful when I am leaving now. Last but not least, I am very thankful and dedicate this thesis to my beloved family who, although very far in distance, are always in my heart. They gave me a huge support, encouragement and everlasting love and trust throughout every stage of my life. iv Declaration This thesis contains no material that has been accepted for the award of any other degree or diploma. To the best of my knowledge, this thesis contains no material previously published or written by another author, except where due reference is made in the text of the thesis. All work presented is primarily that of the author with paper co-authors making contributions to the relevant chapters. Chapter 3 includes all the photometric measurements published in Reda et al., 2004, MN- RAS, 354, 851 and Reda, Forbes & Hau, 2005, MNRAS, 360, 693. The CCD images of galaxies obtained at the ESO/MPG 2.2-m telescope were observed by Ale Terlevich and Duncan Forbes and reduced by Paul Goudfrooij and Vera Kozhurina-Platais. Chapter 4 is an extract from the submitted paper Reda, Proctor, Forbes, Hau & Larsen, 2007, MNRAS, in press. The spectroscopic observations for galaxies and standard stars and basic data reduction including bias, dark current and flat field subtraction, wavelength calibration and correction for the S-distortion and sky subtraction, carried out by G.K.T Hau and D.A.Forbes and published in their paper Hau & Forbes, 2006, MNRAS, 371, 633. Their 2-D spectra are used by the author to produce results discussed in this chapter. The radial profile of the recession velocity in figure 4.3 and velocity dispersion in the top panels of figures 4.4 are based on new measurements by the author and included in this chapter. Chapter 5 is based upon the published paper Reda, Forbes & Hau (2005). The velocity dispersion measurements from Hau & Forbes (2006) are replaced by new measurements by the author from chapter 4. Minor alterations have been made to the published works in order to maintain consistency of style. Fatma M. Reda v vi Contents Abstract i Acknowledgements iv Declaration v Contents vi ListofFigures...................................... xi ListofTables ...................................... xiii 1 Introduction 1 1.1 Generaloverview ................................. 1 1.2 Formationandevolutionscenarios. ...... 3 1.3 Basic properties of early-type galaxies . ......... 6 1.3.1 Surfacebrightnessprofiles . .. 6 1.3.2 Morphologicalfinestructure . .. 7 1.3.3 Internalkinematics . 8 1.3.4 Interstellarmedia . .. .. 9 1.4 Stellarpopulations .............................. .. 10 1.4.1 Stellarpopulationparameters . ... 10 1.4.2 Stellarpopulationcolours . .. 11 1.4.3 Lick/IDSindices ............................. 12 1.4.4 Lick/IDS indices and stellar population parameters . ......... 14 1.4.5 Stellar evolutionary isochrones . ..... 15 vii 1.4.6 Single stellar population models . .... 16 1.4.7 Internal radial gradients of stellar populations . .......... 17 1.5 Scalingrelations................................ .. 18 1.5.1 The surface brightness-size relation . ...... 18 1.5.2 Thefundamentalplane.. .. .. 19 1.5.3 Centralstellarpopulations . ... 20 1.6 Outlineofthethesis .............................. 21 2 Isolated early-type galaxies 23 2.1 Introduction.................................... 23 2.2 Previous studies of early-type galaxies in low-density environments . 23 2.3 Sample selection and the isolation criteria . ......... 26 2.4 Comparisonsample................................ 27 3 Photometric properties 31 3.1 Introduction.................................... 31 3.2 Observationsanddatareduction. ..... 31 3.3 Measuring photometric parameters of the isolated galaxies .......... 32 3.3.1 Magnitudesandcolours . .. .. 34 3.3.2 Modelling and surface brightness profile fitting . ....... 34 3.3.3 Isophotalshapeparameters . 38 3.3.4 Residualimages.............................. 39 3.4 Faintgalaxydetection . .