Alessandro Rettura

Recent Structural Evolution of Early-Type Galaxies: Size Growth from z = 1 to z = 0*

Strong size and internal density evolution of early-type galaxies between -->z ~ 2 and the present has been reported by several authors. Here we analyze samples of nearby and distant ( -->z ~ 1) galaxies with dynamically measured masses in order to confirm the previous, model-dependent results and constrain the uncertainties that may play a role. Velocity dispersion (σ) measurements are taken from the literature for 50 morphologically selected -->0.8 Mdyn = 2 × 1011 M☉. Sizes ( -->Reff) are determined with Advanced Camera for Surveys imaging. We compare the distant sample with a large sample of nearby ( -->0.04 σ − Reff distributions of the nearby and distant samples, regardless of sample selection effects. The implied evolution in -->Reff at fixed mass between -->z = 1 and the present is a factor of -->1.97 ± 0.15. This is in qualitative agreement with semianalytic models; however, the observed evolution is much faster than the predicted evolution. Our results reinforce and are quantitatively consistent with previous, photometric studies that found size evolution of up to a factor of 5 since -->z ~ 2. A combination of structural evolution of individual galaxies through the accretion of companions and the continuous formation of early-type galaxies through increasingly gas-poor mergers is one plausible explanation of the observations.

Formation epochs, star formation histories and sizes of massive early-type galaxies in cluster and field environments at z=1.2: insights from the rest-frame UV

We derive stellar masses, ages and star formation histories of massive early-type galaxies in the z=1.237 RDCS1252.9-2927 cluster and compare them with those measured in a similarly mass-selected sample of field contemporaries drawn from the GOODS South Field. Robust estimates of these parameters are obtained by comparing a large grid of composite stellar population models with 8-9 band photometry in the rest-frame NUV, optical and IR, thus sampling the entire relevant domain of emission of the di!erent stellar populations. Additionall y, we present new, deep U-band photometry of both fields, giving access to the critical FUV rest-frame, in order to constrain empirically the dependence on the environment of the most recent star formation processes. We also analyze the morphological properties of both samples to examine the dependence of their scaling relations on their mass and environment. We find that early-type galaxies, both in the cluster and in the field, show analogous optical morphologies, follow comparable mass vs. size relation, have congruent average surface stellar mass densities and lie on the same Kormendy relation. We also that a fraction of early-type galaxies in the field employ longer timescales, ! , to assemble their mass than their cluster contemporaries. Hence we conclude that, while the formation epoch of early-type only depends on their mass, the environment does regulate the timescales of their star formation histories. Our deep U-band imaging strongly supports this conclusions. I shows that cluster galaxies are at least 0.5 mag fainter than their field contemporaries of similar mass and optical-to-infrared colors, implying that the last episode of star formation must have happened more recently in the field than in the cluster. Subject headings: galaxies: clusters: individual: RDCS1252.9-2927 — galaxies: high-redshift — galaxies: fundamental parameters — galaxies: evolution — galaxies: formation — galaxies: elliptical — cosmology: observations

Evidence for significant growth in the stellar mass of brightest cluster galaxies over the past 10 billion years

Using new and published data, we construct a sample of 160 brightest cluster galaxies (BCGs) spanning the redshift interval 0.03 < z < 1.63. We use this sample, which covers 70 per cent of the history of the universe, to measure the growth in the stellar mass of BCGs after correcting for the correlation between the stellar mass of the BCG and the mass of the cluster in which it lives. We find that the stellar mass of BCGs increases by a factor of 1.8 ± 0.3 between z = 0.9 and z = 0.2. Compared to earlier works, our result is closer to the predictions of semi-analytic models. However, BCGs at z = 0.9, relative to BCGs at z = 0.2, are still a factor of 1.5 more massive than the predictions of these models. Star formation rates in BCGs at z ∼ 1 are generally too low to result in significant amounts of mass. Instead, it is likely that most of the mass build up occurs through mainly dry mergers in which perhaps half of the mass is lost to the intra-cluster medium of the cluster.

Galaxy Clusters around Radio-loud Active Galactic Nuclei at 1.3 < z < 3.2 as Seen by Spitzer

We report the first results from the Clusters Around Radio-Loud AGN program, a Cycle 7 and 8 Spitzer Space Telescope snapshot program to investigate the environments of a large sample of obscured and unobscured luminous radio-loud active galactic nuclei (AGNs) at 1.2 –0.1 (AB), which efficiently selects high-redshift (z > 1.3) galaxies of all types, we identify galaxy cluster member candidates in the fields of the radio-loud AGN. The local density of these Infrared Array Camera (IRAC)-selected sources is compared to the density of similarly selected sources in blank fields. We find that 92% of the radio-loud AGN reside in environments richer than average. The majority (55%) of the radio-loud AGN fields are found to be overdense at a ≥2σ level; 10% are overdense at a ≥5σ level. A clear rise in surface density of IRAC-selected sources toward the position of the radio-loud AGN strongly supports an association of the majority of the IRAC-selected sources with the radio-loud AGN. Our results provide solid statistical evidence that radio-loud AGN are likely beacons for finding high-redshift galaxy (proto-)clusters. We investigate how environment depends on AGN type (unobscured radio-loud quasars versus obscured radio galaxies), radio luminosity and redshift, finding no correlation with either AGN type or radio luminosity. We find a decrease in density with redshift, consistent with galaxy evolution for this uniform, flux-limited survey. These results are consistent with expectations from the orientation-driven AGN unification model, at least for the high radio luminosity regimes considered in this sample.

Cluster galaxies in XMMU J2235-2557: galaxy population properties in most massive environments at z ∼1.4

We present a multi-wavelength study of galaxy populations in the core of the massive, X-ray luminous cluster XMMU J2235 at z=1.39, based on high quality VLT and HST photometry at optical and near-infrared wavelengths. We derive luminosity functions in the z, H, and Ks bands, approximately corresponding to restframe U, R and z band. These show a faint-end slope consistent with being flat, and a character istic magnitude Mclose to passive evolution predictions of Mof local massive clusters, with a formation redshift z> 2. The color-magnitude and color-mass diagrams show evidence of a tight red sequence (intrinsic scatter . 0: 08) of massive galaxies already in place, with overall old stellar populations and g enerally early-type morphology. Beside the red colors, these massive (> 6� 10 10 M�) galaxies typically show early-type spectral features, an d rest-frame far-UV emission consistent with very low star formation rates (SFR< 0: 2M�/yr). Star forming spectroscopic members, with SFR of up to� 100M�/yr, are all located at clustercentric distances &250kpc, with the central cluster region already appearing effectively quenched. Most part of the cluster galaxies more massive than 6� 10 10 Mwithin the studied area do not appear to host significant levels of st ar formation. The high-mass end galaxy populations in the core of this cluster appear to be in a very advanced evolutionary stage, not only in terms of formation of the stellar populations, but also of the asse mbly of the stellar mass. The high-mass end of the galaxy stellar mass function is essentially already in place. The stellar mass f raction estimated within r500 (�1%, Kroupa IMF) is already similar to that of local massive clusters. On the other hand, surface brightness distribution modeling of the massive red sequence galaxies may suggest that their size is often smaller than expected based on the local stellar mass vs size relation. An evolution of the stellar mass vs size relation m ight imply that, in spite of the overall early assembly of these sources , their evolution is not complete, and processes like minor ( and likely dry) merging might still shape the structural properties of thes e objects to resemble those of their local counterparts, wit hout substantially affecting their stellar mass or host stellar populations. None theless, a definite conclusion on the actual relevance of siz e evolution for the studied early-type sample is precluded by possible systematics and biases.

VLT and ACS observations of RDCS J1252.9-2927: Dynamical structure and galaxy populations in a massive cluster at z=1.237

We present results from an extensive spectroscopic survey, carried out with VLT FORS, and from an extensive multiwavelength imaging data set from the HST Advanced Camera for Surveys and ground-based facilities, of the cluster of galaxies RDCS J1252.9-2927. We have spectroscopically confirmed 38 cluster members in the redshift range 1.22 1, kinematic structure. The velocity distribution, which is not Gaussian at the 95% confidence level, is consistent with two groups that are also responsible for the projected east-west elongation of the cluster. The groups are composed of 26 and 12 galaxies with velocity dispersions of 486 and 426 km s-1, respectively. The elongation is also seen in the intracluster gas and the dark matter distribution. This leads us to conclude that RDCS J1252.9-2927 has not yet reached a final virial state. We extend the analysis of the color-magnitude diagram of spectroscopic members to more than 1 Mpc from the cluster center. The scatter and slope of non-[O II]-emitting cluster members in the near-IR red sequence is similar to that seen in clusters at lower redshift. Furthermore, most of the galaxies with luminosities greater than ~K + 1.5 do not show any [O II], indicating that these more luminous, redder galaxies have stopped forming stars earlier than the fainter, bluer galaxies. Our observations provide detailed dynamical and spectrophotometric information on galaxies in this exceptional high-redshift cluster, delivering an in-depth view of structure formation at this epoch only 5 Gyr after the big bang.

The Spitzer Extragalactic Representative Volume Survey (SERVS): Survey Definition and Goals

We present the Spitzer Extragalactic Representative Volume Survey (SERVS), an 18 square degrees medium-deep survey at 3.6 and 4.5 microns with the post-cryogenic Spitzer Space Telescope to ~2 microJy (AB=23.1) depth of five highly observed astronomical fields (ELAIS-N1, ELAIS-S1, Lockman Hole, Chandra Deep Field South and XMM-LSS). SERVS is designed to enable the study of galaxy evolution as a function of environment from z~5 to the present day, and is the first extragalactic survey both large enough and deep enough to put rare objects such as luminous quasars and galaxy clusters at z>1 into their cosmological context. SERVS is designed to overlap with several key surveys at optical, near- through far-infrared, submillimeter and radio wavelengths to provide an unprecedented view of the formation and evolution of massive galaxies. In this paper, we discuss the SERVS survey design, the data processing flow from image reduction and mosaicing to catalogs, as well as coverage of ancillary data from other surveys in the SERVS fields. We also highlight a variety of early science results from the survey.

THE MID-INFRARED ENVIRONMENTS OF HIGH-REDSHIFT RADIO GALAXIES

Taking advantage of the impressive sensitivity of Spitzer to detect massive galaxies at high redshift, we study the mid-infrared environments of powerful, high-redshift radio galaxies at 1.2 –0.1 (AB), in the fields of 48 radio galaxies at 1.2 1.2.

The environmental dependence of the stellar mass function at z ~ 1 - Comparing cluster and field between the GCLASS and UltraVISTA surveys

Aims. We present the stellar mass functions (SMFs) of star-forming and quiescent galaxies from observations of ten rich, red-sequence selected, clusters in the Gemini Cluster Astrophysics Spectroscopic Survey (GCLASS) in the redshift range 0.86 < z < 1.34. We compare our results with field measurements at similar redshifts using data from a K_s-band selected catalogue of the COSMOS/UltraVISTA field. Methods. We construct a K_s-band selected multi-colour catalogue for the clusters in eleven photometric bands covering u-8 μm, and estimate photometric redshifts and stellar masses using spectral energy distribution fitting techniques. To correct for interlopers in our cluster sample, we use the deep spectroscopic component of GCLASS, which contains spectra for 1282 identified cluster and field galaxies taken with Gemini/GMOS. This allowed us to correct cluster number counts from a photometric selection for false positive and false negative identifications. Both the photometric and spectroscopic samples are sufficiently deep that we can probe the SMF down to masses of 10^10 M_⊙. Results. We distinguish between star-forming and quiescent galaxies using the rest-frame U − V versus V − J diagram, and find that the best-fitting Schechter parameters α and M∗ are similar within the uncertainties for these galaxy types within the different environments. However, there is a significant difference in the shape and normalisation of the total SMF between the clusters and the field sample. This difference in the total SMF is primarily a reflection of the increased fraction of quiescent galaxies in high-density environments. We apply a simple quenching model that includes components of mass- and environment-driven quenching, and find that in this picture 45^(+4)_(-3)% of the star-forming galaxies, which normally would be forming stars in the field, are quenched by the cluster. Conclusions. If galaxies in clusters and the field quench their star formation via different mechanisms, these processes have to conspire in such a way that the shapes of the quiescent and star-forming SMF remain similar in these different environments.

The Effects of the Local Environment and Stellar Mass on Galaxy Quenching to z ~ 3

We study the effects of local environment and stellar mass on galaxy properties using a mass complete sample of quiescent and star-forming systems in the COSMOS field at