62 research outputs found
Galaxy evolution as a function of mass and environment: giant and dwarf galaxies in superclusters and in the field
No full textIt 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 (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
The evolution of the galaxy red sequence in simulated clusters and groups
No full textmethods : n-body simulations-galaxies Udgivelsesdato: 24 Jul
Dissecting the spin distribution of dark matter haloes
No full textThe spin probability distribution of dark matter haloes has often been modelled as being very near to a lognormal. Most of the theoretical attempts to explain its origin and evolution invoke some hypotheses concerning the influence of tidal interactions or merging on haloes. Here we apply a very general statistical theorem introduced by Cramér (1936) to study the origin of the deviations from the reference lognormal shape: we find that these deviations originate from correlations between two quantities entering the definition of spin, namely the ratio J/M5/2 (which depends only on mass) and the modulus E of the total (gravitational + kinetic) energy. To reach this conclusion, we have made usage of the results deduced from two high spatial- and mass-resolution simulations. Our simulations cover a relatively small volume and produce a sample of more than 16000 gravitationally bound haloes, each traced by at least 300 particles. We verify that our results are stable to different systematics, by comparing our results with those derived by the GIF2 and by a more recent simulation performed by Macciò et al. We find that the spin probability distribution function shows systematic deviations from a lognormal, at all redshifts z <~ 1. These deviations depend on mass and redshift: at small masses they change little with redshift, and also the best lognormal fits are more stable. The J -M relationship is well described by a power law of exponent ñ very near to the linear theory prediction (ñ = 5/3), but systematically lower than this at z <~ 0.3. We argue that the fact that deviations from a lognormal PDF are present only for high-spin haloes could point to a role of large-scale tidal fields in the evolution of the spin PDF
Stellar population gradients from cosmological simulations: dependence on mass and environment in local galaxies
No full textThe age and metallicity gradients for a sample of group and cluster galaxies from N-body+hydrodynamical simulation are analysed in terms of galaxy stellar mass. Dwarf galaxies show null age gradient with a tail of high and positive values for systems in groups and cluster outskirts. Massive systems have generally zero-age gradients which turn to positive for the most massive ones. Metallicity gradients are distributed around zero in dwarf galaxies and become more negative with mass; massive galaxies have steeper negative metallicity gradients, but the trend flattens with mass. In particular, fossil groups are characterized by a tighter distribution of both age and metallicity gradients. We find a good agreement with both local observations and independent simulations. Interestingly, our results suggest that environment differently affects the gradients at low and high masses. The results are also discussed in terms of the central age and metallicity, as well as the total colour, specific star formation and velocity dispersion
A study on the multicolour evolution of red-sequence galaxy populations: Insights from hydrodynamical simulations and semi-analytical models
No full textContext. By means of our own cosmological-hydrodynamical simulation (SIM) and semi-analytical model (SAM), we studied galaxy population properties in clusters and groups, spanning over ten different bands from the ultraviolet to the near-infrared (NIR), and their evolution since redshift z = 2. Aims. We compare our results in terms of red/blue galaxy fractions and of the luminous-to-faint ratio (LFR) on the red sequence (RS) with recent observational data reaching beyond z = 1.5. Methods. Different selection criteria were tested to retrieve the galaxies that effectively belong to the RS: either by their quiescence degree measured from their specific star formation rate (sSFR; the so-called "dead sequence"), or by their position in a colour-colour plane, which is also a function of sSFR. In both cases, the colour cut and the lower limit magnitude thresholds were let to evolve with redshift so that they would follow the natural shift of the characteristic luminosity in the luminosity function (LF). Results. We find that the Butcher-Oemler effect is wavelength-dependent, with the fraction of blue galaxies increasing more steeply in optical-optical than in NIR-optical colours. Moreover, a steep trend in the blue fraction can only be reproduced when an optically fixed luminosity-selected sample is chosen, while the trend flattens when selecting samples by stellar mass or by an evolving magnitude limit. We also find that the RS-LFR behaviour, highly debated in the literature, is strongly dependent on the galaxy selection function: in particular, the very mild evolution that is recovered when using a mass-selected galaxy sample agrees with values reported for some of the highest redshift-confirmed (proto)clusters. For differences that are attributable to environments, we find that normal groups and (to a lesser extent) cluster outskirts present the highest values of both the star-forming fraction and LFR at low z, while fossil groups and cluster cores have the lowest values: this separation among groups begins after z ~ 0.5, while at earlier epochs all groups share similar star-forming properties. Conclusions. Our results support a picture where star formation is still active in SIM galaxies at redshift 2, in contrast with SAM galaxies, which have formed earlier and are already quiescent in cluster cores at that epoch. Over the whole interval considered, we also find that the more massive RS galaxies from the mass-selected sample grow their stellar mass at a higher rate than less massive ones. On the other hand, no dearth of red dwarfs is reported at z 1 from either model
Evolution of the mass-metallicity relations in passive and star-forming galaxies from SPH-cosmological simulations
No full textWe present results from SPH-cosmological simulations, including self-consistent modeling of supernova feedback and chemical evolution, of galaxies belonging to two clusters and 12 groups. We reproduce the mass-metallicity (ZM) relation of galaxies classified in two samples according to their star-forming (SF) activity, as parameterized by their specific star formation rate (sSFR), across a redshift range up to z = 2. The overall ZM relation for the composite population evolves according to a redshift-dependent quadratic functional form that is consistent with other empirical estimates, provided that the highest mass bin of the brightest central galaxies is excluded. Its slope shows irrelevant evolution in the passive sample, being steeper in groups than in clusters. However, the subsample of high-mass passive galaxies only is characterized by a steep increase of the slope with redshift, from which it can be inferred that the bulk of the slope evolution of the ZM relation is driven by the more massive passive objects. The scatter of the passive sample is dominated by low-mass galaxies at all redshifts and keeps constant over cosmic times. The mean metallicity is highest in cluster cores and lowest in normal groups, following the same environmental sequence as that previously found in the red sequence building. The ZM relation for the SF sample reveals an increasing scatter with redshift, indicating that it is still being built at early epochs. The SF galaxies make up a tight sequence in the SFR-M * plane at high redshift, whose scatter increases with time alongside the consolidation of the passive sequence. We also confirm the anti-correlation between sSFR and stellar mass, pointing at a key role of the former in determining the galaxy downsizing, as the most significant means of diagnostics of the star formation efficiency. Likewise, an anti-correlation between sSFR and metallicity can be established for the SF galaxies, while on the contrary more active galaxies in terms of simple SFR are also metal-richer. Finally, the [O/Fe] abundance ratio is presented too: we report a strong increasing evolution with redshift at given mass, especially at z ≳ 1. The expected increasing trend with mass is recovered when only considering the more massive galaxies. We discuss these results in terms of the mechanisms driving the evolution within the high- and low-mass regimes at different epochs: mergers, feedback-driven outflows, and the intrinsic variation of the star formation efficiency. © 2013. The American Astronomical Society. All rights reserved.
Astrocomp: a web portal for high performance computing on a grid of supercomputers
No full textAstrocomp is a project based on a collaboration among the University of Roma La Sapienza, the Astrophysical Observatory of Catania and ENEA. The main motivation of the AstroComp project is to construct a portal, which allows to set up a repository of computational codes and common databases, making them available and enjoyable, with a user-friendly graphical web interface, to the international community. AstroComp will allow the scientific community to benefit by the use of many different numerical tools implemented on high performance computing (HPC) resources, both for theoretical astrophysics and cosmology and for the storage and analysis of astronomical data, without the need of specific training, know-how and experience either in computational techniques or in database construction and management methods. An essential feature of Astrocomp is that it makes available to subscribers some CPU time on large parallel platforms, via specific grants. Astrocomp is partly financed by a grant of the Italian national research Council (CNR)
Environment or outflows? New insight into the origin of narrow associated QSO absorbers
No full textRecent detailed studies of Narrow Absorption Line (NAL) systems in QSO-spectra have revealed that at least 50% of QSOs have NALs associated with the central engine, and in most cases they are found to be outflowing. Will studies of NALs provide the much sort after evidence for ubiquitous QSO feedback that can halt the formation of stars in galaxies? I present new results on the distribution of line-of-sight velocity offsets between MgII absorbers and their background QSOs, based on a large catalogue of absorbers from SDSS DR6 and greatly improved QSO-redshift estimates. My analysis reveals a high-velocity population of MgII NALs extending out to at least 6000 km/s from the QSOs, which cannot be ascribed to the clustering of local galaxies, similar to that observed recently for CIV absorbers. The very existence of such low ionisation gas clouds in the intense ionising field of the QSO suggests that we may indeed be witnessing the mechanical expulsion of gas, alongside the heating previously observed. I also show that there is a significant excess of low-velocity MgII NALs in radio-loud QSOs compared to radio-quiet QSOs. In the near future, improved QSO clustering results will allow us to say whether this is due to environmental or feedback effects
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