Max Spolaor. Presented in fulfillment of the requirements of the degree of Doctor of Philosophy. September 2009

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1 Radial Gradients in Elliptical Galaxies Max Spolaor Presented in fulfillment of the requirements of the degree of Doctor of Philosophy September 2009 Faculty of Information and Communication Technology Swinburne University

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3 Abstract i The aim of this Thesis is to provide a contribution to the decades-long debate regarding the formation and evolution of early-type galaxies. Our approach to this open problem is to investigate the combined kinematic, photometric, and stellar population properties at large galactocentric radii for a sample of early-type galaxies. The galactocentric radial distribution of these properties is a chemodynamical imprint of the many physical mechanisms acting in galaxies, and provides us with strong constraints on competing galaxy formation scenarios. Initially we the derive the star formation and chemical enrichment history of two massive early-type galaxies. Our analysis is based on new high signal-to-noise long-slit spectroscopic data obtained from the ESO 3.6m telescope, and high-resolution multiband imaging data from the Hubble Space Telescope and wide-field imaging from the Subaru telescope. We derive stellar population radial profiles of age, metallicity [Z/H], and α-element abundance ratio [α/fe] out to more than one effective radius, together with surface brightness profiles and isophotal shape parameters. The results suggest that the galaxies formed over half of their mass in a single short-lived burst of star formation at high redshift and evolved quiescently afterwards. This event likely involved an outside-in mechanism with supernova-driven galactic winds playing a fundamental role in shaping the observed steep negative radial metallicity gradients. A similar study is performed out to 1 3 effective radii for a sample of 14 lowluminosity, low-mass early-type galaxies in the Fornax and Virgo clusters. We use new high-quality long-slit spectroscopic data obtained from the Gemini telescope and multiband imaging data from the Hubble Space Telescope. A gradual gas dissipation is suggested to be responsible for the old and extended stellar discs present in these galaxies. We extend our study to higher galaxy mass via a novel literature compilation of 37 early-type galaxies, which provides stellar population properties out to one effective radius. We find that metallicity gradients correlate with galactic mass, and the relationship shows a sharp change in slope at a dynamical mass of M. We conclude that low-luminosity, low-mass galaxies likely formed in an early starforming collapse with extended, low efficiency star formation, and mass-dependent galactic outflows of metal-enriched gas. Luminous, high-mass galaxies might have formed initially by mergers of gas-rich disc galaxies and then subsequently evolved via dry merger events.

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5 Acknowledgements iii Writing the acknowledgements section of this Thesis has given me the opportunity to realise what a great and rewarding adventure my PhD studies have been, starting from arriving Down Under (i.e., Australia) three years ago. I would like to express my first and biggest thanks to my three supervisors Duncan Forbes, Warrick Couch and George Hau each of which has contributed in different ways. Duncan has spent the last three years guiding, advising and supporting me in the hard task of setting my path in the astronomical research world. I deeply appreciate his help, without which I could never have made it. Warrick has been the source of many lively discussions over scientific problems, and I am very grateful of the time that he has always found for me despite his busy schedule. George has provided me with advice, encouragement and support which made me reach the end. I have enjoyed working out the details of many problems with him. Special thanks go to Robert Proctor and Chiaki Kobayashi. The help, support and scientific input that they have offered me has gone well beyond the call of duty. It has been a pleasure working with them. The next thanks are for all the people that have helped me in discussing and interpreting results: Sarah Brough, Alister Graham, Darren Croton, Glen Mackie, Antonio Pipino, Trevor Mendel, Lee Spitler, Sarah Burke-Spolaor, Fatma Reda, Caroline Foster, Alan Brito. Honourable mention is required to my fellow students of the second and third floor of the Centre for Astrophysics & Supercomputing, that have made my time here in Melbourne very enjoyable: Lina Levin, Joris Verbiest, Adrian Malec, Stefan Oslowski, Emily Wisnioski, Francesco Pignatale, Ben Barsdell, Christima Blom, Evelyn Caris, Carlos Contreras, Andrew Green, Adam Deller, Annie Hughes, Peter Jensen, Simon Mutch, Catarina Ubach. Special acknowledgments go to Rob Crain, Glenn Kacprzak, Chris Fluke, Chris Blake, Matthew Owers, Greg Poole, Willem van Straten, Richard Wilman, Max Bernyk, Mark McAuley and Matthew Bailes. I can not forget to mention my friends outside the astronomical world for their constant encouragement and support: Simone Cataldi, Nick Jones, David and Stefania Verzegnassi, Ben Cumming, Marco Fogal, Enrico Pettarin, and Walter Boschin. Last but not least I would like to thank my wife Sarah Burke-Spolaor and my extended Family. Sarah has been my source of strength and motivation to keep moving forward despite the everyday troubles. I deeply appreciate her invaluable help in reading and

6 iv checking my english grammar of my long papers. I would like to wish her good luck in finishing her PhD in Astrophysics. My parents, Maurizio and Cinzia, and my sisters, Clizia and Stella, have been my foundation columns for many years. Their constant encouragement, their constant supporting of my choices, and their constant just being there for a Skype phone call between different time zones have made the difference for me. Thanks everybody!

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8 vi Statement of originality The work presented in this thesis has been carried out in the Centre for Astrophysics & Supercomputing at the Swinburne University of Technology between 2006 and 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. The content of the Chapters listed below has appeared in refereed journals. Minor alterations have been made to the published papers in order to maintain argument continuity and consistency of spelling and style. Chapter 2 has been published as The early-type galaxies NGC 1407 and NGC I. Spatially resolved radial kinematics and surface photometry in 2008, MNRAS, 385, 667 by Max Spolaor, Duncan A. Forbes, George K. T. Hau, Robert N. Proctor, and Sarah Brough. Chapter 3 has been published as The early-type galaxies NGC 1407 and NGC II. Star formation and chemical evolutionary history in 2008, MNRAS, 385, 675 by Max Spolaor, Duncan A. Forbes, Robert N. Proctor, George K. T. Hau, and Sarah Brough. Chapter 4 has been submitted to MNRAS as Early-type galaxies at large galactocentric radii - I. Stellar kinematics and photometric properties by Max Spolaor, George K. T. Hau, Duncan A. Forbes, and Warrick J. Couch. Chapter 5 has been submitted to MNRAS as Early-type galaxies at large galactocentric radii - II. Metallicity gradients, and the [Z/H] mass, [α/fe] mass relations by Max Spolaor, Chiaki Kobayashi, Duncan A. Forbes, Warrick J. Couch, and George K. T. Hau. The discovery of the mass-metallicity gradient relation has been published as The Mass-Metallicity Gradient Relation of Early-Type Galaxies in 2009, ApJL, 691, 138 by Max Spolaor, Robert N. Proctor, Duncan A. Forbes, Warrick J. Couch. Max Spolaor Melbourne, Australia September 2009

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11 Contents Abstract Acknowledgements Declaration List of Figures List of Tables i ii v xii xiv 1 Introduction Purpose of the Thesis An introduction to early-type galaxies Isophotal and kinematic properties Stellar populations Stellar population parameters derived from Lick/IDS line-strength indices Stellar population properties The Fundamental Plane Galaxy formation and evolution scenarios Monolithic collapse Hierarchical merging Thesis outline NGC 1407 and NGC 1400: kinematics and photometry Introduction The data sample Observations and data reduction ESO/EFOSC2 spectroscopic data HST/ACS imaging data Suprime-Cam imaging data Spatially resolved radial kinematics Kinematic analysis NGC 1407 results NGC 1400 results Spatially resolved surface photometry ix

12 x Contents Surface photometry modelling Surface brightness profile fitting Isophotal shape parameters and fine structure The peculiar velocity of NGC Summary and Conclusions NGC 1407 and NGC 1400: star formation and chemical evolution Introduction The data sample Observations and data reduction Spatially resolved stellar populations Stellar population model fitting procedure Central stellar population parameters Stellar population radial profiles Observed and predicted colour index radial profiles Discussion Conclusions Early-type galaxies at large galactocentric radii - I Introduction The data sample Observations and data reduction GMOS long-slit spectroscopy HST/ACS imaging Galaxy stellar kinematics Radial profiles Galaxy surface photometry Radial profiles Individual galaxies FCC 148 (NGC 1375) FCC 153 (IC 1963) FCC 170 (NGC 1381) FCC 277 (NGC 1428) FCC 301 (ESO 358-G059) FCC 335 (ESO 359-G002) VCC 575 (NGC 4318)

13 Contents xi VCC 828 (NGC 4387) VCC 1025 (NGC 4434) VCC 1146 (NGC 4458) VCC 1178 (NGC 4464) VCC 1297 (NGC 4486B) VCC 1475 (NGC 4515) VCC 1630 (NGC 4551) Galaxy properties v rot /σ radial profiles Rotational support versus Anisotropy Rotational support versus Discyness/Boxiness LOSVD deviations versus Rotational support Discussion Conclusions Early-type galaxies at large galactocentric radii - II Introduction The data sample Low-mass galaxies High-mass galaxies Observations and data reduction GMOS long-slit spectroscopy Data reduction Stellar population analysis Calibrations to the Lick/IDS system Emission-line correction Stellar population model-fitting Radial profiles of ages, metallicities, and abundance ratios Galaxy properties Stellar population gradients Central and mean stellar population properties Relations between stellar population parameters Individual galaxies FCC 148 (NGC 1375) FCC 153 (IC 1963) FCC 170 (NGC 1381)

14 xii Contents FCC 277 (NGC 1428) FCC 301(ESO 358-G059) FCC 335 (ESO 359-G002) VCC 575 (NGC 4318) VCC 828 (NGC 4387) VCC 1025 (NGC 4434) VCC 1146 (NGC 4458) VCC 1178 (NGC 4464) VCC 1297 (NGC 4486B) VCC 1475 (NGC 4515) VCC 1630 (NGC 4551) Discussion Origin of metallicity gradients Do the [Z/H]-mass and [α/fe]-mass relations have a common origin? Summary and Conclusions Conclusions and future directions Conclusions Future directions Bibliography 132 A Appendix A 151 B Appendix B 157 C Appendix C 159 D Appendix D 165

15 List of Figures 1.1 Stellar mass function of early-type galaxies Star formation histories as function of mass and environment Large-scale major-axis kinematics of NGC Large-scale major-axis kinematics of NGC Spatially resolved radial photometric parameters of NGC NGC 1407 photometric modelling parameters Spatially resolved radial photometric parameters of NGC NGC 1400 photometric modelling parameters The spectrum of NGC 1407 and NGC The average χ deviation of Lick/IDS indices Stellar population radial profiles of NGC 1407 and NGC Observed and predicted colour profiles The major-axis stellar kinematics of the Fornax cluster galaxies The major-axis stellar kinematics of the Virgo cluster galaxies The major-axis stellar kinematics of the Virgo cluster galaxies Isophotal properties of the Fornax cluster galaxies Isophotal properties of the Virgo cluster galaxies Isophotal properties of the Virgo cluster galaxies v rot /σ radial profiles of the Fornax cluster galaxies v rot /σ radial profiles of the Virgo cluster galaxies The v max / σ ǫ eff diagram The log(v/σ) M B diagram The log(v/σ) B 4 diagram The local h 3 v rot /σ relation The local h 4 v rot /σ relation (v max /σ 0 ) frequency distributions B 4 frequency distributions Stellar population radial profiles of the Fornax cluster low-mass galaxies Stellar population radial profiles of the Virgo cluster low-mass galaxies Stellar population radial profiles of the Virgo cluster low-mass galaxies Stellar population gradients xiii

16 xiv List of Figures 5.5 The mass-metallicity gradient relation as a function of environment and central age Stellar population parameters as a function of central stellar velocity dispersion Histograms of central and mean values of stellar population parameters Relations between central values of stellar population parameters Observed and predicted mass-metallicity gradient relation Observed and predicted mass trends

17 List of Tables 1.1 Lick/IDS index definitions Data sample properties Distance measurements of NGC 1407 and NGC Instrumental configuration Kinematic measurements of NGC 1407 and NGC Surface brightness fitting parameters of NGC 1407 and NGC Galaxy isophote modelling parameters of NGC 1407 and NGC Data sample properties Instrumental configuration Central stellar population parameters for NGC 1407 and NGC Stellar population radial gradients Observed and predicted colour radial gradients Observed and predicted C [Z/H] ratio Data sample properties Global kinematic and photometric parameters Low-mass data sample High-mass data sample - A High-mass data sample - B Offsets for the observed standard Lick/IDS stars Values of stellar population gradients Statistics of stellar population properties Central values of stellar population parameters Mean values of stellar population parameters Best-fitting parameters to our central and mean values of stellar population parameters versus mass relations Literature compilation of best-fitting slopes to central values of stellar population parameters versus mass relations A.1 Stellar kinematic radial parameters of NGC A A.2 Stellar kinematic radial parameters of NGC B A.3 Stellar kinematic radial parameters of NGC A A.4 Stellar kinematic radial parameters of NGC B xv

18 xvi List of Tables B.1 Offsets between our index measurements and published values of the five Lick/IDS and spectrophotometric standard stars C.1 Stellar population radial parameters of NGC A C.2 Stellar population radial parameters of NGC B C.3 Stellar population radial parameters of NGC A C.4 Stellar population radial parameters of NGC B

19 1 Introduction 1.1 Purpose of the Thesis How do early-type galaxies form? What is the chemodynamical evolutionary history of early-type galaxies? What are the physical mechanisms responsible for the spatial distribution of their stellar population properties? The answers to these questions, among the many other interrogatives concerning the origin of early-type galaxies, still remain an open debate in the astronomical community. Although progress has been made on both the theoretical and observational points of view, a number of competing formation and evolution models exist. The aim of this work is to provide observational constraints on the physical mechanisms acting in early-type galaxies. The main focus is on the varying properties with galactocentric radius in early-type galaxies, because the existence of such radial gradients and their relationship with other galaxy structural parameters provides strong leverage in distinguishing competing galaxy formation scenarios. Our contribution works to enhance the understanding of the formation and evolution of galaxies, and takes us closer to a definitive answer of the above mentioned questions. 1.2 An introduction to early-type galaxies The term early-type galaxies refers to galaxies that are morphologically classified as ellipticals and lenticulars, in contrast to spirals, which instead are called late-type galaxies. The origin of this classification scheme lies in the incorrect interpretation of the Hubble tuning fork diagram (Hubble 1936) as an evolutionary sequence. Specifically, ellipticals and lenticulars do not move down the forks of the diagram, converting into spiral galaxies as they evolve. Morphology does, however, correlate with the stellar content and colours 1

20 2 Chapter 1. Introduction Figure 1.1 The stellar mass function of early-type galaxies (red crosses) and late-type galaxies (blue squares) derived by Baldry et al. (2004) from a large sample of low-redshift galaxies from the SDSS. Best-fitting Schechter functions are shown as solid black lines. The dotted lines represent the stellar mass functions derived by Bell et al. (2003) using SDSS and 2MASS. The figure is from Renzini (2006). of galaxies such that early-type galaxies are redder and show prevalent absorption features and very weak emission lines in their spectra. Bell et al. (2003) and Baldry et al. (2004) have found that at least 60 percent of the total stellar mass in the local Universe belongs to early-type galaxies, although they represent only 17 percent of the total number of galaxies in the sample. In Fig. 1.1 (as presented in Renzini 2006) we show the results of these separate studies on the stellar mass function of colour-selected early-type galaxies in a large sample of low-redshift galaxies from the Sloan Digital Sky Survey (SDSS) and the Two Micron All Sky Survey (2MASS). From the plot it is evident that for stellar masses larger than M, the number density of early-type galaxies becomes increasingly dominant over late-type galaxies. Moreover, galaxies less massive than M (i.e., dwarfs) contribute less than 20 percent of the net stellar mass of early-type galaxies. The distinctive features observed in the stellar population, stellar isophotes, internal

21 1.3. Isophotal and kinematic properties 3 kinematics, and structure of early-type galaxies are interpreted as the chemodynamical fossil imprints of the many physical formation and evolution mechanisms taking place within a galaxy. The different processes during galaxy formation are coupled in a complex way and their efficiency is predicted to vary with galactocentric radius leaving measurable changes at different radii. Observationally, we can disentangle their effect by focusing not just on a particular category of properties (e.g., stellar kinematics) but performing a combined analysis, and by probing galactic regions beyond the galaxy s nucleus. These features make early-type galaxies excellent laboratories for testing galaxy formation models. In the sections below I will provide a summary of the properties of early-type galaxies from recent research in this field. 1.3 Isophotal and kinematic properties The surface brightness profiles of early-type galaxies are well fit, to a first approximation, by a r 1/4 power law (de Vaucouleurs 1959). The essential parameters describing this light profile are the effective radius (r e, defined as the radius within which half of the galaxy light is contained), and the surface brightness within the effective radius (µ(r e )). However, the high resolution Hubble Space Telescope profiles have shown that stellar light profiles at radii interior to 1 kpc deviate from the inward extrapolation of the de Vaucouleurs law (e.g., Lauer et al. 1995). Profiles flatter than a r 1/4 power law regime are termed core-like profiles, while steeper profiles produce a central cusp. To parametrise the HST profiles, the Nuker law and more recently the core-sersic law have been proposed as ad hoc analytical formulas (e.g., Ferrarese et al. 1994; Lauer et al. 1995; Graham et al. 2003; Trujillo et al. 2004). These two categories of nuclei coincide with other galaxies properties (Faber et al. 1997), thus adding to a long-recognised division of the early-type galaxy population (e.g., Kormendy & Bender 1996; Kormendy et al. 2009). In general, core-like surface brightness profiles are found in luminous, massive earlytype galaxies (i.e., M B 20.5). Studies of the kinematics of these galaxies show that they are slowly rotating, triaxial systems supported by the pressure of anisotropic stellar orbits, but with a significant amount of minor-axis rotation (e.g., Binney 1978; Binney & Tremaine 1988; Bender et al. 1992). Although contributing to only a few percent of the total galaxy light, the central galactic regions can occasionally host kinematically decoupled cores (e.g., Franx & Illingworth 1988; Kormendy 1982). Isophotal analysis of these galaxies reveals a small degree of ellipticity of their isophotes, with boxy deviations from a perfect ellipsoid. In other words, an excess of light lies along a line at 45 with respect to the galaxy s major and minor axis. Shells and ripples are often present in the

22 4 Chapter 1. Introduction residual images of these galaxies (e.g., Forbes & Thomson 1992). Radio activity and strong X-ray emission from diffuse hot gas components in a large halos (typical size of 5 10r e ) are often detected (e.g., Bender et al. 1989) Intermediate ( 19.0 M B 20.5) and low-luminosity, low-mass early-type galaxies (i.e., M B 19) have cuspy central profiles. The galaxies are fast rotators, and they can be well described as isotropic, oblate spheroids. The large amount of rotational support contribute to their degree of flatness (e.g., Bender & Nieto 1990; Bender et al. 1994). Kinematically decoupled cores are rarely found in these galaxies, which instead can contain extended stellar discs. Their isophotes are disc-shaped, or in other terms an excess of light along the galaxy s major and/or minor axis is detected, and the amount of disciness correlates with the amount of rotational support (e.g., Binney 1976; Davies et al. 1983; Bender 1988). In contrast to giant galaxies, these objects display low radio activity and X-ray emission (e.g., Bender et al. 1987; Goudfrooij & Trinchieri 1998). Differences in the observed properties of early-type galaxies has led to the idea of a dichotomy into two families, which was ultimately interpreted as evidence of different galaxy formation histories (e.g., Faber et al. 1997; Côté et al. 2007; Kormendy et al. 2009). Discy, rapidly rotating, cuspy early-type galaxies are considered to be remnants of gas-rich, spiral-spiral mergers, where the central cusp is an excess of extra light due to mergerinduced central star formation (e.g., Hopkins et al. 2009a). Early-type galaxies with core profiles, slow rotation, and boxy-shaped isophotes form in gas-poor mergers. Stars in the central regions are scattered out by the coalescence of two supermassive binary black holes induced by the merging of their host galaxies, thus producing the core-like surface brightness profile. The progenitors of these galaxies could be cuspy early-type galaxies that have exhausted their gas reservoir (e.g., Hopkins et al. 2009b). 1.4 Stellar populations Pioneering studies have attempted to analyse the stellar population of galaxies via the interpretation of their observed integrated colours. Early-type galaxies are found to have a typical B V colour index of mag, that is consistent with the colour of G-K dwarf and giant stars in our Galaxy (e.g., Peletier et al. 1990). This prompted the idea that early-type galaxies mainly consist of old stellar populations, and age effects have only minor influence on their colours. The colours of early-type galaxies correlate with their luminosity, such that more luminous galaxies tend to be redder. Under the assumption of old stellar ages, the metallicity is interpreted to be the main driving parameter of the slope of the colour-magnitude rela-

23 1.4. Stellar populations 5 tion (CMR; e.g., Sandage & Katem 1977). The slope of the CMR is found to be constant to high redshifts, and the scatter of the relation is instead attributed to a scatter in age (e.g., Bower et al. 1992; Bender et al. 1996; Kodama et al. 1998). Over the past three decades, these findings have led to ideas that early-type galaxies may form at high-redshift in a short burst of star formation, and the stellar populations of more luminous galaxies are more metal-rich because their deeper gravitational potential well is able to stop metals from escaping outside the galaxy. In general, the central regions of early-type galaxies are observed to be redder than the outer regions (e.g., Peletier & Valentijn 1989). If colours are assumed to be purely an effect of metallicity, then colour gradients imply gradients in metallicity. The presence of metallicity gradients in early-type galaxies could be explained as a consequence of metal-enriched gas sinking towards the galaxy centre, thus fuelling the production of central metal-rich stars. Colour-magnitude diagrams, and colour indices can provide some information on the average metallicity of stellar populations in early-type galaxies. However, the derived metallicities are not entirely reliable due to the sensitivity of stellar colours to both age and metallicity. This is found to be particularly problematic for optical colours, which are known to be completely degenerate with regard to variations in age and metallicity (e.g., Bruzual & Charlot 1993; Bell et al. 2000). The situation is further complicated by the fact that colours represent composite stellar populations and systematic uncertainties can be introduced by variations in element abundance ratios and by dust reddening. The advent of the Lick/IDS system of absorption spectral features allowed a quantification of the age/metallicity degeneracy, thus breaking the degeneracy and revolutionising the study of stellar population from integrated light of early-type galaxies Stellar population parameters derived from Lick/IDS line-strength indices The Lick/IDS system is a set of 25 absorption lines in the wavelength range Å, defined from the spectra of a large survey of Galactic stars observed between 1972 and 1984 with the Image Dissector Scanner mounted on the 3m Shane Telescope at the Lick Observatory. The evolution of the Lick/IDS system, starting from the original 11 indices (Faber et al. 1985) to the final index definitions of the 25 absorption features (Trager et al. 1998), is reported in a series of publications ( Burstein et al. 1986; Gorgas et al. 1993; Worthey et al. 1994a; Worthey & Ottaviani 1997). In Table 1.1, we show the final list of the 25 Lick/IDS indices as defined in Trager et al. (1998). The index definition includes the bandpass of each absorption feature and

24 6 Chapter 1. Introduction two wavelength regions on either side of the feature of interest to determine a pseudocontinuum level. Eighteen indices are indicative of atomic absorption lines and they are measured in Å of equivalent width. The remaining seven indices measure the molecular bands CN 2, CN 1, Mg 1, Mg 2, Mg b, TiO 1, and TiO 2, and they are expressed in magnitudes of absorbed flux. Worthey (1994b) used the range of sensitivities to age and metallicity effects displayed by the Lick indices to quantify and break the age/metallicity degeneracy. The degeneracy is known as the 3/2 rule, where log Age/ log Z 3/2, so that a factor of three age change produces the same spectral change as a factor two change in metallicity. Age, metallicity, and α-element abundance ratios are the parameters that describe the stellar population of a galaxy. Stellar ages can provide insight on the epoch of star formation episodes in a galaxy. The total metallicity [Z/H] expresses the sum of all elements (Z) heavier than helium relative to hydrogen (H). We use the standard notation [Z/H] = log(z/h) log(z /H ) to indicate the total stellar metallicity. Similarly, the α-element abundance ratio [α/fe] refers to the mass of a specific group of elements with respect to that of the Fe-peak elements. The importance of distinguishing between [Z/H] and [α/fe] is that light α-elements and heavy Fe-peak elements are produced in two different types of supernovae. The so-called α-elements N, O, Ca, Na, Ne, S, Si, and Ti are mainly generated by Type II supernovae, which explode when thermonuclear burning ceases in the core of the most massive stars. Since the progenitors of Type II supernovae are rapidly evolving massive stars, these supernovae occur in the early stages of star formation episodes (e.g., Arimoto et al. 1997). The Fe-peak elements Cr, Mn, Fe, Co, Ni, Cu and Zn are produced in Type Ia supernovae, which are exploding white dwarfs in binary systems. The progenitors are low-mass stars with longer lifetimes, and therefore these supernovae occur in the late stages of star formation (e.g., Greggio & Renzini 1983). The time-delay in the occurrence of Type Ia and Type II supernovae and their relative contribution in the metals production is such that the [α/fe] abundance ratio is often used as indicator of star formation duration (e.g., Matteucci & Greggio 1986; Matteucci 1994). A short burst of star formation produces stars with high [α/fe] ratios, because the gas out of which they are formed has not been pre-enriched by Fe-peak elements from Type Ia supernovae (e.g., Thomas 1999b). In extended star-forming episodes, the metal contribution from Type Ia supernovae becomes important such that new stars are formed out of metal enriched gas and therefore have lower [α/fe] ratios (e.g., Thomas et al. 2005). The measurement of these stellar population parameters from integrated galaxy light is made possible by comparing the observed Lick/IDS indices to single stellar population

25 1.4. Stellar populations 7 Index Resolution [Å] Index Bandpass [Å] Pseudo-continuum [Å] Units H δa Å H δf Å CN mag CN mag Ca Å G Å H γa Å H γf Å Fe Å Ca Å Fe Å C Å H β Å Fe Å Mg mag Mg mag Mg b Å Fe Å Fe Å Fe Å Fe Å Fe Å Na D Å TiO mag TiO mag Table 1.1 Lick/IDS index definitions as originally reported in Trager et al. (1998)

26 8 Chapter 1. Introduction (SSP) models. These models are an example of evolutionary population synthesis, and they assume that all stars are coeval, share the same chemical composition, and their mass distribution is given by a specific initial mass function (e.g., Maraston 2005). Worthey (1994b) defined a set of empirical fitting functions to describe the correlations of the Lick/IDS line-strength indices to stellar effective temperature T eff, metallicity [Fe/H] and surface gravity log(g). By using these empirical fitting of polynomials, they were able to convert the stellar atmospheric parameters (T eff, [Fe/H], log(g)) of theoretical isochrones into single stellar population values of observables such as the Lick/IDS indices. However, an important limitation of this technique is that the fitting functions incorporate the same abundance ratios that are present in the stellar library. The calibrating stars are in the solar neighbourhood of the Milky Way disc, thus the abundance pattern is biased towards high [α/fe] ratios at low-metallicities (e.g., McWilliam 1997). In the past years, a number of studies have produced different stellar population models (e.g., Vazdekis 2001; Bruzual & Charlot 2003; Thomas et al. 2003). In this Thesis we make use of the models of Thomas et al. (2003, 2004), because they are able to reproduce the effects of varying α-abundance ratios on Lick/IDS indices. In the models, a well-defined [α/fe] ratio is assigned at each metallicity by increasing the abundance of the α-elements and by decreasing that of the Fe-peak elements such that total metallicity is preserved. The SSPs used to build the stellar population models of Thomas et al. (2003, 2004) are from Maraston (1998) and Maraston et al. (2003). The SSPs are based on the stellar isochrones of Bono et al. (1997), Cassisi et al. (2000) and Salasnich et al. (2000), with an initial mass function set to be a Salpeter slope (Salpeter 1955). The variation of the Lick/IDS indices due to element abundance changes is computed by using an extension (Korn et al. 2005) of the synthetic fitting functions of Tripicco & Bell (1995). The calibration of the models is carried out with Milky Way globular clusters, because they closely resemble a single stellar population and their stellar population content can be observationally measured, independently of any models (e.g., Puzia et al. 2002). The range of ages, metallicities and α-element abundance ratios that different globular clusters display is found to well cover that of early-type galaxies (e.g., Mendel et al. 2007). The models of Thomas et al. (2003, 2004) provide SSP values for the whole set of Lick/IDS indices. For each index, a 3-dimensional grid of model Age, [Z/H], and [α/fe] values are provided. Each index in the model SSPs is interpolated to give a grid of values for 1 < Age < 15 Gyr, 2.25 < [Z/H] < 0.67 dex in steps of dex, and 0.3 < [α/fe] < 0.6 dex in 0.03 dex steps (e.g., Proctor et al. 2004a).

27 1.4. Stellar populations Stellar population properties Many studies have analysed the stellar population content of early-type galaxies by measuring a small number of Lick/IDS indices (e.g., Jorgensen 1999; Kuntschner et al. 2001; Poggianti et al. 2001; Trager et al. 2000b; Caldwell et al. 2003; Mehlert et al. 2003; Nelan et al. 2005; Bernardi et al. 2006). The age/metallicity degeneracy is broken by plotting appropriate age-sensitive indices against metallicity-sensitive indices. Age and metallicity values are then obtained by the comparison of these diagnostic diagrams with predictions from stellar population synthesis models. The most widely used indices for diagnostic diagrams have been Mg b, Fe (the average of Fe5270 and Fe5335), and H β. An important discovery is that central regions of bright early-type galaxies present high values of metallicity-sensitive indices and an excess of Mg 2 with respect to Fe (e.g., Davies et al. 1987; Worthey et al Gorgas et al. 1997). The studies showed that high [Mg 2 /Fe] (e.g., [α/fe]) can be achieved if the star formation timescale is 1 Gyr, and the gas out of which the observed stars are formed has been pre-enriched. Specifically, short bursts of star formation ensure that Type II supernovae dominate the metal enrichment of the interstellar medium from which the bulk of stars is created. Low-luminosity early-type galaxies have almost solar values of central Mg 2 and Fe, suggesting a long star-forming episode and therefore a major metal contribution from Type Ia supernovae. The correlation that Mg 2 and Fe show with stellar velocity dispersion σ, led to the interpretation that galactic outflows of metals due to galactic winds are more effective in low-mass galaxies because of their shallower gravitational potential wells. As a consequence, massive galaxies are more metal-rich than low-mass galaxies. Furthermore, the steeper slope of the Mg 2 -σ relation with respect to Fe -σ prompted the conclusion that [Mg 2 /Fe] (e.g., [α/fe]) correlates with stellar velocity dispersion. In other words, the star formation timescale is mass dependent. Several studies (e.g., Jorgensen 1999; Trager et al. 2000b; Kuntschner et al. 2001) have concluded that not all early-type galaxies are old, but rather that they exhibit a range of ages from a few to almost 15 Gyr, and that field galaxies tend to be younger than early-type galaxies in clusters. The age-sensitive index H β is found to be anti-correlated with σ, such that stronger lines are observed in low-mass galaxies. Thus, massive galaxies are older than less massive objects. Trager et al. (2000b) questioned the validity of H β as age indicator because of its sensitivity to low levels of recent star formation. They showed that a small frosting of centrally concentrated young stars could make a galaxy appear younger in the diagnostic diagrams despite having an old overall stellar population. The work of Thomas et al. (2005) on 125 early-type galaxies in low- and high-density

28 10 Chapter 1. Introduction environments contributed to a greater understanding of stellar population properties. Instead of focusing on stellar population values of individual galaxies, they searched for statistical patterns in the various diagnostic diagrams. The reliability of the results was tested by comparing the observed pattern to those simulated in mock galaxy samples generated via Monte Carlo realisations. In performing these simulations particular emphasis was given to a comprehensive modelling of error propagation. The main result of this work can be summarised by a set of three equations: log Age/Gyr = 0.46(0.17) (0.32) log σ, [Z/H] = 1.06( 1.03) (0.57) log σ, [α/fe] = 0.42( 0.42) (0.28) log σ, (1.1) describing the correlations of age, metallicity, and α-element abundance ratio with central velocity dispersion (i.e, galaxy mass) in low-density (quantities in parenthesis) and high-density (quantities not in parenthesis) environments. The star formation scenario suggested by these equations is summarised in Fig. 1.2 from Thomas et al. (2005). Massive galaxies experience a short ( 1 Gyr), intense burst of star formation at redshift z > 3. The peak of star formation moves towards lower redshifts for less massive galaxies, and contemporaneously the starburst duration becomes more extended. For example, galaxies of mass as low as 10 9 M can experience an almost constant, low level of star formation for more than 4 Gyr. This mass-dependent star formation scenario is found to be the same in low- and high-density environments, the main difference being that the peak of star formation activity for galaxies in low-density environments is delayed by almost 2 Gyr with respect to early-type galaxies in high-density environments. The results of Thomas et al. (2005) have been confirmed by further recent studies that have considered larger numbers of early-type galaxies. However, the relationships that early-type galaxies are found to have are based on observations of just the nuclear regions (i.e., r e /8) of galaxies. These galactic regions encompass only a small amount ( 15 percent) of the total stellar mass and therefore are not indicative of the stellar population content of the whole galaxy. The existence of stellar population radial gradients was firstly suggested by observations of gradients in the metallicity-sensitive Mg 2 and Fe indices (e.g., Carollo et al. 1993; Davies et al. 1993). The spatial distribution of the age-sensitive H β index shows no indication of radial gradients. Mehlert et al. (2003) confirmed the presence of significant negative radial metallicity gradients in a sample of 35 early-type galaxies in the Coma cluster. Age and [α/fe] radial gradients are found to be very small on average. This is in

29 1.5. The Fundamental Plane 11 Figure 1.2 The star formation scenario for early-type galaxies proposed by Thomas et al. (2005). Average star formation histories of galaxies as a function of their stellar mass M and environment. The stellar masses range from M to M and correspond to velocity dispersion σ = 100, 140, 190, 240, 280, 320 km s 1. contrast with the significant age gradients found by Sánchez-Blázquez et al. (2007) in a sample of field, group and cluster early-type galaxies. In this Thesis we explore the stellar population gradients of early-type galaxies and how their correlations with other galaxy properties can be used to differentiate between competing galaxy formation scenarios. By quantifying the slope of the gradients, we are also able to assess the effects that they produce on the stellar population scaling relations derived from the central galactic regions. 1.5 The Fundamental Plane The effective radius r e, the effective surface brightness µ e (or luminosity L = µ e 2πre) 2 and the velocity dispersion σ of early-type galaxies are confined to a tight plane in threedimensional parameter space (e.g., Djorgovski & Davis 1987; Dressler This empirical correlation is known as fundamental plane (FP) and is described by the equation: log r e = αlog σ + βµ e + γ (1.2)

30 12 Chapter 1. Introduction where α, β, and γ are the best-fitting parameters. The projection of the FP on to the r e µ e plane produces the Kormendy relation (Kormendy 1977), while the Faber-Jackson relation (Faber & Jackson 1976) is obtained from the projection onto the σ L plane. The argument used to explain the small scatter observed in the edge-on projections of the plane is based on the interpretation given for the narrowness of the CM relation (e.g. Lucey et al. 1991; Bower et al. 1992; Renzini & Ciotti 1993). The scatter is proposed to be due to a small age dispersion of galaxies that formed the bulk of their stars at high redshift. The physical explanation of the FP invokes the virial theorem under the assumption of a constant mass-to-light ratio (M/L) and isotropic velocity distributions. However, this interpretation is challenged by the observed change in slope of the FP. This tilt is such that the M/L ratio appears to increase by a factor of 3 along the plane, while the mass increases by a factor of 100. This variation does not imply a change in the assumption of galaxies to be virialised systems. The effects of stellar population properties (i.e., age and metallicity) appear to explain only half of the apparent variation of the M/L ratio (e.g., Proctor et al. 2008). The rest of the tilt could be explained by some dependence of dynamical support and structural properties on the luminosity (e.g., Bender et al. 1992; Busarello et al. 1997). Many studies have looked for variations of the scatter and zero-point of the FP as a function of redshift and environment (e.g., Franx 1999; Bernardi et al. 2003). No significant variation in the driving parameters of the FP or in the scatter has been found, leading to the suggestion of a high-redshift formation scenario of galaxies, with consequently passive evolution. The FP and its small scatter implies a high homogeneity in the formation and evolution mechanisms acting in early-type galaxies. Although such regularity is a natural outcome of an early star-forming collapse, a further explanation for such properties could be the presence of centrally concentrated gas in the merging events experienced by early-type galaxies during their formation. 1.6 Galaxy formation and evolution scenarios Theoretical scenarios that describe the formation and evolution of early-type galaxies include: a revised version of monolithic collapse, and the dissipative (wet/gas-rich) and dissipationless (dry/gas-poor) merger alternatives of hierarchical clustering.

31 1.6. Galaxy formation and evolution scenarios Monolithic collapse In the classical models of monolithic collapse (or early star-forming collapse; Eggen et al. 1962; Larson 1974a; Larson 1975; Carlberg 1984; Arimoto & Yoshii 1987), stars form in all regions during a rapid collapse and remain in their orbits, whereas the gas dissipates to the centre of the galaxy. The shape and ellipticity of stellar orbits are expected to depend on the initial angular momentum of the collapsing protogalactic gas cloud and on the effectiveness of gas viscosity in redistributing angular momentum towards outer radii during the collapse. The sinking gas is continuously enriched by evolving stars. As a consequence, stars formed in the centre are predicted to be more metal-rich than those born in the outer galaxy regions. The efficiency of this process is proportional to the depth of the galactic potential well, so that a strong correlation between a galaxy s metallicity gradient and mass is expected. The evolving stars are also responsible for the α-element enrichment of the interstellar medium (ISM). To reproduce the supersolar abundance of α-elements with respect to the iron-peak elements observed in early-type galaxies, models require that the collapse and star forming process occur on timescales 1 Gyr, which will produce null or very small age gradients (Arimoto & Yoshii 1987; Matteucci 1994; Thomas 1999b). Recently, feedback processes such as supernova-driven galactic winds have been re-considered in more detailed numerical models of monolithic collapse (Larson 1974b; Arimoto & Yoshii 1987; Gibson 1997; Martinelli et al. 1998; Chiosi & Carraro 2002; Kawata & Gibson 2003; Pipino & Matteucci 2006a; Pipino & Matteucci 2008a). It is suggested that they may create and shape the abundance gradients. Galactic winds initiate when the energy injected by supernova explosions balances the gravitational binding energy of the ISM. The winds evacuate the gas, eliminating the essential fuel for any future star formation. The shallower local potential well of the external galactic regions supports the early development of winds with respect to the central regions. Therefore, in central galactic regions the star formation and the chemical enrichment last longer. Dissipation of gas towards the galaxy centre and a time-delay in the occurrence of galactic winds cooperate in steepening any metallicity gradient. In summary, dissipative collapse models predict: i) very steep negative metallicity gradients, that strongly correlate with galaxy mass (Chiosi & Carraro 2002; Kawata & Gibson 2003), ii) small or null age gradients, as a consequence of the short timescales involved in the collapse and star formation processes, iii) α-element enhancement gradients that can be either positive, negative or null (Martinelli et al. 1998; Pipino et al. 2006b), due to the interplay between local differences in the star formation timescale and gas flows (Pipino et al. 2008b), iv) isophotal shape and ellipticity proportional to the protogalactic