Galaxy evolution as a function of mass and environment: giant and dwarf galaxies in superclusters and in the field
Creators: name: Gargiulo, Adriana
Publisher: name: Università degli Studi di Napoli Federico II
Year: 2008

description: 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 &amp; 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 &amp; Zabludoff
2005; Rines et al. 2005; Baldry et al. 2006; Blanton, Berlind &amp; Hogg 2007;
Boselli &amp; Gavazzi 2006; Haines et al. 2006a; Mercurio et al. 2006; Sorrentino,
Antonuccio-Delogo &amp; 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 &amp; 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 &amp; 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 (&lt;2Gyr) of star-formation at z&gt;2 (Chiosi &amp; 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 &amp; Rees 1978; Kauffmann, White &amp; Guideroni 1993;
Lacey &amp; 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 &amp; 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 &gt; 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 &amp; 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 &amp; Neistein 2007). In particular,
the transition from cold to hot accretion modes of gas when galaxy halos
reach a mass ∼1012M (Dekel &amp; 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&lt;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.