The Different Environmental Dependencies of Star-formation for Giant and Dwarf Galaxies

We examine the origins of the bimodality observed in the global properties of galaxies around a stellar mass of 3x10^10 M_sun by comparing the environmental dependencies of star-formation for the giant and dwarf galaxy populations. The Sloan Digital Sky Survey DR4 spectroscopic dataset is used to produce a sample of galaxies in the vicinity of the supercluster centered on the cluster A2199 at z=0.03 that is ~90% complete to a magnitude limit of M*+3.3. From these we measure global trends with environment for both giant (M_r<-20 mag) and dwarf (-19<M_r<-17.8 mag) subsamples using the luminosity-weighted mean stellar age and H_alpha emission as independent measures of star-formation history. The fraction of giant galaxies classed as old (t>7 Gyr) or passive (EW[H_alpha]<4 A) falls gradually from ~80% in the cluster cores to ~40% in field regions beyond 3-4 R_virial, as found in previous studies. In contrast, we find that the dwarf galaxy population shows a sharp transition at ~1 R_virial, from being predominantly old/passive within the cluster, to outside where virtually all galaxies are forming stars and old/passive galaxies are only found as satellites to more massive galaxies. These results imply fundamental differences in the evolution of giant and dwarf galaxies: whereas the star-formation histories of giant galaxies are determined primarily by their merger history, star-formation in dwarf galaxies is much more resilient to the effects of major mergers. Instead dwarf galaxies become passive only once they become satellites within a more massive halo, by losing their halo gas reservoir to the host halo, or through other environment-related processes such as galaxy harassment and/or ram-pressure stripping.Comment: 4 pages, 4 figures, accepted for publication in ApJ

Global Properties of the Rich Cluster ABCG 209 at z~0.2. Spectroscopic and Photometric Catalogue

This paper is aimed at giving an overview of the global properties of the rich cluster of galaxies ABCG 209. This is achieved by complementing the already available data with new medium resolution spectroscopy and NIR photometry which allow us to i) analyse in detail the cluster dynamics, distinguishing among galaxies belonging to different substructures and deriving their individual velocity distributions, using a total sample of 148 galaxies in the cluster region, of which 134 belonging to the cluster; ii) derive the cluster NIR luminosity function; iii) study the Kormendy relation and the photometric plane of cluster early-type galaxies (ETGs). Finally we provide an extensive photometric (optical and NIR) and spectroscopic dataset for such a complex system to be used in further analyses investigating the nature, formation and evolution of rich clusters of galaxies. The observational scenario confirms that ABCG 209 is presently undergoing strong dynamical evolution with the merging of two or more subclumps. This interpretation is also supported by the detection of a radio halo (Giovannini et al. 2006) suggesting that there is a recent or ongoing merging. Cluster ETGs follow a Kormendy relation whose slope is consistent with previous studies both at optical and NIR wavelengths. We investigate the origin of the intrinsic scatter of the photometric plane due to trends of stellar populations, using line indices as indicators of age, metallicity and alpha/Fe enhancement. We find that the chemical evolution of galaxies could be responsible for the intrinsic dispersion of the Photometric Plane.Comment: 39 pages, 17 figures, MNRAS in pres

ACCESS II: A Complete Census of Star Formation in the Shapley Supercluster - UV and IR Luminosity Functions

We present panoramic Spitzer/MIPS mid- and far-infrared and GALEX ultraviolet imaging of the the most massive and dynamically active system in the local Universe, the Shapley supercluster at z=0.048, covering the 5 clusters which make up the supercluster core. We combine these data with existing spectroscopic data from 814 confirmed supercluster members to produce the first study of a local rich cluster including both ultraviolet and infrared luminosity functions (LFs). This joint analysis allows us to produce a complete census of star-formation (both obscured and unobscured), extending down to SFRs~0.02-0.05Msun/yr, and quantify the level of obscuration of star formation among cluster galaxies, providing a local benchmark for comparison to ongoing and future studies of cluster galaxies at higher redshifts with Spitzer and Herschel. The GALEX NUV and FUV LFs obtained have steeper faint-end slopes than the local field population, due largely to the contribution of massive, quiescent galaxies at M_FUV>-16. The 24um and 70um galaxy LFs for the Shapley supercluster instead have shapes fully consistent with those obtained for the Coma cluster and for the local field galaxy population. This apparent lack of environmental dependence for the shape of the FIR luminosity function suggests that the bulk of the star-forming galaxies that make up the observed cluster infrared LF have been recently accreted from the field and have yet to have their star formation activity significantly affected by the cluster environment. We estimate a global SFR of 327 Msun/yr over the whole supercluster core, of which just ~20% is visible directly in the UV continuum and ~80% is reprocessed by dust and emitted in the infrared. The level of obscuration (L_IR/L_FUV) in star-forming galaxies is seen to increase linearly with L_K over two orders of magnitude in stellar mass.Comment: 19 pages, 17 figures. Accepted for publication in MNRA

New insights into the structure of early-type galaxies: the Photometric Plane at z~0.3

We study the Photometric Plane (PHP), namely the relation between the effective radius re, the mean surface brightness within that radius e, and the Sersic index n, in optical (R and I) and near-infrared (K) bands for a large sample of early-type galaxies (ETGs) in the rich cluster MS1008-1224 at z=0.306. The PHP relation has an intrinsic dispersion of ~32% in re, and turns out to be independent of waveband. This result is consistent with the fact that internal colour gradients of ETGs can have only a mild dependence on galaxy luminosity (mass). There is no evidence for a significant curvature in the PHP. We show that this can be explained if this relation origins from a systematic variation of the specific entropy of ETGs along the galaxy sequence, as was suggested from previous works. The intrinsic scatter of the PHP is significantly smaller than for other purely photometric relations, such as the Kormendy relation and the photometric Fundamental Plane, which is constructed by using colours in place of velocity dispersions. The scatter does not depend on the waveband and the residuals about the plane do not correlate with residuals of the colour-magnitude relation. Finally, we compare the coefficients of the PHP at z~0.3 with those of ETGs at z~0, showing that the PHP is a valuable tool to constrain the luminosity evolution of ETGs with redshift. The slopes of the PHP do not change significantly with redshift, while the zero-point is consistent with cosmological dimming of the surface brightness in an expanding universe plus the passive fading of galaxy stellar populations with a high formation redshift (z_f >1-2).Comment: 21 pages, 10 figures, MNRAS in pres

Galaxy evolution as a function of environment and luminosity

We present an analysis of star formation and nuclear activity of about 28000 galaxies in a volume-limited sample taken from SDSS DR4 low-redshift catalogue (LRC) taken from the New York University Value Added Galaxy Catalogue (NYU-VAGC) of Blanton et al. 2005, with 0.005<z<0.037, ~90\% complete to M_r=-18.0. We find that in high-density regions ~70 per cent of galaxies are passively evolving independent of luminosity. In the rarefied field, however, the fraction of passively evolving galaxies is a strong function of luminosity, dropping from 50 per cent for Mr <~ -21 to zero by Mr ~ -18. Moreover the few passively evolving dwarf galaxies in field regions appear as satellites to bright (>~ L*) galaxies. Moreover the fraction of galaxies with the optical signatures of an active galactic nucleus (AGN) decreases steadily from ~50\% at Mr~-21 to ~0 per cent by Mr~-18 closely mirroring the luminosity dependence of the passive galaxy fraction in low-density environments (see fig. 1 continuous lines). This result reflects the increasing importance of AGN feedback with galaxy mass for their evolution, such that the star formation histories of massive galaxies are primarily determined by their past merger history.Comment: Proceedings of the Workshop held in Vulcano (Messina), Italy, May 18-22 200

Transformations of galaxies in the environments of the cluster ABCG 209 at z~0.2

We analyse the properties of galaxy populations in the rich Abell cluster ABCG 209 at redshift z~0.21, on the basis of spectral classification of 102 member galaxies. We take advantage of available structural parameters to study separately the properties of bulge-dominated and disk-dominated galaxies. The star formation histories of the cluster galaxy populations are investigated by using line strengths and the 4000 A break, through a comparison to stellar population synthesis models. The dynamical properties of different spectral classes are examined in order to infer the past merging history of ABCG 209. The cluster is characterized by the presence of two components: an old galaxy population, formed very early (z_f >~ 3.5), and a younger (z$_f >~$ 1.2) population of infalling galaxies. We find evidence of a merger with an infalling group of galaxies occurred 3.5-4.5 Gyr ago. The correlation between the position of the young H_delta-strong galaxies and the X-ray flux shows that the hot intracluster medium triggered a starburst in this galaxy population ~ 3 Gyr ago.Comment: 20 pages, 9 figures, A&A in pres

LoCuSS: Connecting the Dominance and Shape of Brightest Cluster Galaxies with the Assembly History of Massive Clusters

We study the luminosity gap, dm12, between the first and second ranked galaxies in a sample of 59 massive galaxy clusters, using data from the Hale Telescope, HST, Chandra, and Spitzer. We find that the dm12 distribution, p(dm12), is a declining function of dm12, to which we fitted a straight line: p(dm12) propto -(0.13+/-0.02)dm12. The fraction of clusters with "large" luminosity gaps is p(dm12>=1)=0.37+/-0.08, which represents a 3sigma excess over that obtained from Monte Carlo simulations of a Schechter function that matches the mean cluster galaxy luminosity function. We also identify four clusters with "extreme" luminosity gaps, dm12>=2, giving a fraction of p(dm12>=2)=0.07+0.05-0.03. More generally, large luminosity gap clusters are relatively homogeneous, with elliptical/disky brightest cluster galaxies (BCGs), cuspy gas density profiles (i.e. strong cool cores), high concentrations, and low substructure fractions. In contrast, small luminosity gap clusters are heterogeneous, spanning the full range of boxy/elliptical/disky BCG morphologies, the full range of cool core strengths and dark matter concentrations, and have large substructure fractions. Taken together, these results imply that the amplitude of the luminosity gap is a function of both the formation epoch, and the recent infall history of the cluster. "BCG dominance" is therefore a phase that a cluster may evolve through, and is not an evolutionary "cul-de-sac". We also compare our results with semi-analytic model predictions based on the Millennium Simulation. None of the models are able to reproduce all of the observational results, underlining the inability of current models to match the empirical properties of BCGs. We identify the strength of AGN feedback and the efficiency with which cluster galaxies are replenished after they merge with the BCG in each model as possible causes of these discrepancies. [Abridged]Comment: 15 pages, 12 figures, accepted for publication in MNRA

Probing galaxy evolution through the internal colour gradients, the Kormendy relations and the Photometric Plane of cluster galaxies at z~0.2

We present a detailed analysis of the photometric properties of galaxies in the cluster \A2163B at redshift z~0.2. R-, I- and K-band structural parameters, (half light radius r_e, mean surface brightness _e within r_e and Sersic index n) are derived for N~60 galaxies, and are used to study their internal colour gradients. For the first time, we use the slopes of optical-NIR Kormendy relations to study colour gradients as a function of galaxy size, and we derive the Photometric Plane at z~0.2 in the K band. Colour gradients are negligible at optical wavelengths, and are negative in the optical-NIR, implying a metallicity gradient in galaxies of ~0.2 dex per radial decade. The analysis of the Kormendy relation suggests that its slope increases from the optical to the NIR, implying that colour gradients do not vary or even do become less steep in more massive galaxies. Such a result is not simply accomodated within a monolithic collapse scenario, while it can be well understood within a hierarchical merging framework. Finally, we derive the first NIR Photometric Plane at z~0.2, accounting for both the correlations on the measurement uncertainties and the selection effects. The Photometric Plane at z~0.2 is consistent with that at z~0, with an intrinsic scatter significantly smaller than the Kormendy relation but larger than the Fundamental Plane.Comment: 18 pages, 12 figures, A&A in pres

ACCESS III: The Nature of Star Formation in the Shapley Supercluster

We present a joint analysis of panoramic Spitzer/MIPS mid-infrared and GALEX ultraviolet imaging of the Shapley supercluster at z=0.048. Combining this with spectra of 814 supercluster members and 1.4GHz radio continuum maps, this represents the largest complete census of star-formation (both obscured and unobscured) in local cluster galaxies to date, reaching SFRs~0.02Msun/yr. We take advantage of this comprehensive panchromatic dataset to perform a detailed analysis of the nature of star formation in cluster galaxies, using several quite independent diagnostics of the quantity and intensity of star formation to develop a coherent view of the types of star formation within cluster galaxies. We observe a robust bimodality in the infrared (f_24/f_K) galaxy colours, which we are able to identify as another manifestation of the broad split into star-forming spiral and passive elliptical galaxy populations seen in UV-optical surveys. This diagnostic also allows the identification of galaxies in the process of having their star formation quenched as the infrared analogue to the UV "green valley" population. The bulk of supercluster galaxies on the star-forming sequence have specific-SFRs consistent with local field specific-SFR-M* relations, and form a tight FIR-radio correlation confirming that their FIR emission is due to star formation. We show that 85% of the global SFR is quiescent star formation within spiral disks, as manifest by the observed sequence in the IRX-beta relation being significantly offset from the starburst relation of Kong et al. (2004), while their FIR-radio colours indicate dust heated by low-intensity star formation. Just 15% of the global SFR is due to nuclear starbursts. The vast majority of star formation seen in cluster galaxies comes from normal infalling spirals who have yet to be affected by the cluster environment.Comment: 17 pages, 9 figures. Accepted for publication in MNRA

The Environmental Dependencies of Star-formation and the Origin of the Bimodality in Galaxy Properties

We examine the origins of the bimodality observed in the global properties of galaxies by comparing the environmental dependencies of star-formation for giant and dwarf galaxy populations. Using Sloan Digital Sky Survey (SDSS) DR4 spectroscopic data to create a volume-limited sample complete to M*+3, we find that the environmental dependences of giant and dwarf galaxies are quite different, implying fundamental differences in their evolution. Whereas the star-formation histories of giant galaxies are determined primarily by their merger history, resulting in passively-evolving giant galaxies being found in all environments, we show that this is not the case for dwarf galaxies. In particular, we find that old or passive dwarf galaxies are only found as satellites within massive halos (clusters, groups or giant galaxies), with none in the lowest density regions. This implies that star-formation in dwarf galaxies must be much more resilient to the effects of mergers, and that the evolution of dwarf galaxies is primarily driven by the mass of their host halo, through effects such as suffocation, ram-pressure stripping or galaxy harassment.Comment: 4 pages, 1 figure, to appear in the proceedings of "Cosmic Frontiers", Durham, August 200