Galaxy evolution as a function of mass and environment: giant and dwarf galaxies in superclusters and in the field
Gargiulo, Adriana (2008) Galaxy evolution as a function of mass and environment: giant and dwarf galaxies in superclusters and in the field. [Tesi di dottorato] (Inedito)
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It has been known for decades that local galaxies can be broadly divided into two distinct populations (e.g. Hubble 1926, 1936; Morgan 1958; de Vaucouleurs 1961). The first population consists in red, passively-evolving, bulge-dominated galaxies mainly populated by old stars that, in the colourmagnitude diagram, makes up the “red sequence”, while the second population makes up the “blue cloud” of young, star-forming, disk-dominated galaxies (e.g. Strateva et al. 2001; Kauffmann et al. 2003a,b; Blanton et al. 2003a; Baldry et al. 2004; Driver et al. 2006; Mateus et al. 2006). It has also long been known that the environment in which a galaxy inhabits has a profound impact on its evolution in terms of defining both its structural properties and star-formation histories (e.g. Hubble & Humason 1931). In particular, passively-evolving spheroids dominate cluster cores, whereas in field regions galaxies are typically both star-forming and diskdominated (Blanton et al. 2005a). These differences have been quantified through the classic morphology–density (Dressler 1980) and star-formation (SF)–density relations (Lewis et al. 2002; G´omez et al. 2003). However, despite much effort (e.g. Treu et al. 2003; Balogh et al. 2004a,b; Gray et al. 2004; Kauffmann et al. 2004; Tanaka et al. 2004; Christlein & Zabludoff 2005; Rines et al. 2005; Baldry et al. 2006; Blanton, Berlind & Hogg 2007; Boselli & Gavazzi 2006; Haines et al. 2006a; Mercurio et al. 2006; Sorrentino, Antonuccio-Delogo & Rifatto 2006a; Weinmann et al. 2006a,b; Mateus et al. 2007), it still remains unclear whether these environmental trends are: (i) the direct result of the initial conditions in which the galaxy forms, whereby massive galaxies are formed earlier preferentially in the highest overdensities in the primordial density field, and have a more active merger history, than galaxies that form in the smoother low-density regions; or (ii) produced later by the direct interaction of the galaxy with one or more aspects of its environment, whether that be other galaxies, the intracluster medium, or the underlying dark-matter distribution. Several physical mechanisms have been proposed that could cause the transformation of galaxies through interactions with their environment such as ram-pressure stripping (Gunn & Gott 1972), galaxy harassment (Moore et al. 1996), and suffocation (also known as starvation or strangulation), in which the diffuse gas in the outer galaxy halo is stripped preventing further accretion onto the galaxy before the remaining cold gas in the disk is slowly consumed through star-formation (Larson, Tinsley & Caldwell 1980). The morphologies and star-formation histories of galaxies are also strongly dependent on their masses, with high-mass galaxies predominately passivelyevolving spheroids, and low-mass galaxies generally star-forming disks. A sharp transition between these two populations is found about a characteristic stellar mass of ∼3 × 1010M, corresponding to ∼M+ 1 (Kauffmann et al. 2003a,b). This bimodality implies fundamental differences in the formation and evolution of high- and low-mass galaxies. The primary mechanism behind this transition appears to be the increasing efficiency and rapidity with which gas is converted into stars for more massive galaxies according to the Kennicutt-Schmidt law (Kennicutt 1998; Schmidt 1959). This results in massive galaxies with their deep potential wells consuming their gas in a short burst (<2Gyr) of star-formation at z>2 (Chiosi & Cararro 2002), while dwarf galaxies have much more extended star-formation histories and gas consumption time-scales longer than the Hubble time (van Zee 2001). In the monolithic collapse model for the formation of elliptical galaxies this naturally produces the effect known as “cosmic downsizing” whereby the major epoch of star-formation occurs earlier and over a shorter period in the most massive galaxies and progressively later and over more extended timescales towards lower mass galaxies. This has been confirmed observationally both in terms of the global decline of star-formation rates in galaxies since z∼1 (Noeske et al. 2007a,b) and the fossil records of low-redshift galaxy spectra (Heavens et al. 2004; Panter et al. 2007). Finally, in analyses of the absorption lines of local quiescent galaxies, the most massive galaxies are found to have higher mean stellar ages and abundance ratios than their lower mass counterparts, confirming that they formed stars earlier and over shorter time-scales (Thomas et al. 2005; Nelan et al. 2005). In this scenario, the mass-scale at which a galaxy becomes quiescent should decrease with time, with the most massive galaxies becoming quiescent earliest, resulting in the red sequence of passively-evolving galaxies being built up earliest at the bright end (Tanaka et al. 2005). However, the standard paradigm for the growth of structure and the evolution of massive galaxies within a CDM universe is the hierarchical merging scenario (e.g. White & Rees 1978; Kauffmann, White & Guideroni 1993; Lacey & Cole 1993) in which massive elliptical galaxies are assembled through the merging of disk galaxies as first proposed by Toomre (1977) (see also Struck 2005). Although downsizing appears at first sight to be at odds with the standard hierarchical model for the formation and evolution of galaxies, Merlin & Chiosi (2006) are able to reproduce the same downsizing as seen in the earlier “monolithic” models in a hierarchical cosmological context, resulting in what they describe as a revised monolithic scheme whereby the merging of substructures occurs early in the galaxy life (z > 2). Further contributions to cosmic downsizing and the observed bimodality in galaxy properties could come from the way gas from the halo cools and flows onto the galaxy (Dekel & Birnboim 2006; Kereˇs et al. 2005) and which affects its ability to maintain star-formation over many Gyrs, in conjunction with feedback effects from supernovae and AGN (e.g. Springel et al. 2005a; Croton et al. 2006). These mechanisms which can shut down star-formation in massive galaxies allow the hierarchical CDM model to reproduce very well the rapid early formation and quenching of stars in massive galaxies (e.g. Bower et al. 2006; Hopkins et al. 2006a; Birnboim, Dekel & Neistein 2007). In particular, the transition from cold to hot accretion modes of gas when galaxy halos reach a mass ∼1012M (Dekel & Birnboim 2006) could be responsible for the observed sharp transition in galaxy properties with mass. If the evolution of galaxies due to internal processes occurs earlier and more rapidly with increasing mass, then this would give less opportunity for external environmental processes to act on massive galaxies. Moreover, lowmass galaxies having shallower potential wells could be more susceptible to disruption and the loss of gas due to external processes such as ram-pressure stripping and tidal interactions. This suggests that the relative importance of internal and external factors on galaxy evolution and on the formation of the SF-, age- and morphology-density relations could be mass-dependent, in particular the relations should be stronger for lower mass galaxies. Such a trend has been observed by Smith et al. (2006) who find that radial age gradients (out to 1Rvir) are more pronounced for lower mass (σ<175kms−1) cluster red sequence galaxies than their higher mass subsample. With all this in mind, we undertook the work presented in this thesis studying galaxy evolution as a function of mass and environment (chapter 1). To this aim, we investigate the evolution of giant and dwarf galaxies in cluster environment (Part I) through the analysis of i) luminosity function and colour distribution (chapter 3), and ii) the fundamental plane of early-type galaxies (chapter 4). We extend, then, our analysis to a wide spread of environments, from the rarefied field to the high density regions, (Part II, chapters 6 and 7). This analysis allowed us to discriminate among the possible physical mechanisms which, driving the star-formation of giant and dwarf galaxies, are able to reproduce the observed bimodal galaxy distribution. Technical aspects of the dataset used throughout the present analysis are presented in chapters 2 and 5.
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