N. Menci

Star formation and mass assembly in high redshift galaxies

Aims. The goal of this work is to infer the star formation properties and the mass assembly process of high redshift (0.3 ≤ z 0.3, the star formation rate is correlated well with stellar mass, and this relationship seems to steepen with redshift if one relies on IR-based estimates of the SFR; b) the contribution to the global SFRD by massive galaxies increases with redshift up to � 2.5, more rapidly than for galaxies of lower mass, but appears to flatten at higher z; c) despite this increase, the most important contributors to the SFRD at any z are galaxies of about, or immediately lower than, the characteristic stellar mass;

The K20 survey - I. Disentangling old and dusty star-forming galaxies in the ERO population

We present the results of VLT optical spectroscopy of a complete sample of 78 EROs with R-Ks\geq5 over a field of 52 arcmin^2. About 70% of the 45 EROs with Ks\leq19.2 have been spectroscopically identified with old passively evolving and dusty star-forming galaxies at 0.7<z<1.5. The two classes are about equally populated and for each of them we present and discuss the average spectrum. From the old ERO average spectrum and for Z=Z_{\odot} we derive a minimum age of \sim 3 Gyr, corresponding to a formation redshift of z_f \gtsima 2.4. PLE models with such formation redshifts well reproduce the density of old EROs (consistent with being passively evolving ellipticals), whereas the predictions of the current hierarchical merging models are lower than the observed densities by large factors (up to an order of magnitude). From the average spectrum of the star-forming EROs we estimate a substantial dust extinction with E(B-V) \gtsima 0.5. The star formation rates, corrected for the average reddening, suggest a significant contribution from EROs to the cosmic star-formation density at z \sim 1.

The Abundance of Distant and Extremely Red Galaxies: The Role of AGN Feedback in Hierarchical Models

We investigate the effect of AGN feedback associated with the bright QSO phase on the color distribution of galaxies from z = 0 up to z = 4. To this aim, we insert a blast-wave model of AGN feedback in our semianalytic model of galaxy formation, which includes the growth of supermassive black holes and the AGN activity triggered by interactions of the host galaxies. The AGN feedback is directly related to the impulsive, luminous quasar phase. We test our model by checking the consistency of its results against (1) the QSO luminosity functions from z = 0 to 4, and (2) the observed local relation between the black hole mass mBH and the mass of the host galaxy. At low redshift the inclusion of AGN feedback enhances the number of red bright galaxies so that the color distribution of Mr 1.5) galaxies; at 0.5 2.5.

Faint high-redshift AGN in the Chandra deep field south: the evolution of the AGN luminosity function and black hole demography

Context. We present detection and analysis of faint X-ray sources in the Chandra deep field south (CDFS) using the 4 Ms Chandra observation. Aims. We place constraints on active galactic nuclei (AGN) luminosity functions at z = 3–7, its cosmological evolution, and highredshift black hole and AGN demography. Methods. We use a new detection algorithm, using the entire three-dimensional data-cube (position and energy), and searching for X-ray counts at the position of high-z galaxies in the GOODS-South survey. Results. This optimized technique results in the identification of 54 AGN at z > 3, 29 of which are new detections. Applying stringent completeness criteria, we derive AGN luminosity functions in the redshift bins 3–4, 4–5, and >5.8 and for 42.75 3( 18 +17 −10 %). Their optical counterparts do not show any reddening and we thus conclude that the size of the X-ray absorber is likely smaller than the dust sublimation radius. We finally report the discovery of a highly star-forming galaxy at z = 3.47, arguing that its X-ray luminosity is likely dominated by stellar sources. If confirmed, this would be one of the farthest objects in which stellar sources have been detected in X-rays.

The K20 survey. III. Photometric and spectroscopic properties of the sample

The K20 survey is an ESO VLT optical and near-infrared spectroscopic survey aimed at obtaining spectral infor- mation and redshifts of a complete sample of about 550 objects to Ks 20:0 over two independent fields with a total area of 52 arcmin 2 . In this paper we discuss the scientific motivation of such a survey, we describe the photometric and spectroscopic properties of the sample, and we release the Ks-band photometric catalog. Extensive simulations showed that the sample is photometrically highly complete to Ks= 20. The observed galaxy counts and the R Ks color distribution are consistent with literature results. We observed spectroscopically 94% of the sample, reaching a spectroscopic redshift identification complete- ness of 92% to Ks 20:0 for the observed targets, and of 87% for the whole sample (i.e. counting also the unobserved targets). Deep spectroscopy was complemented with multi-band deep imaging in order to derive tested and reliable photometric red- shifts for the galaxies lacking spectroscopic redshifts. The results show a very good agreement between the spectroscopic and the photometric redshifts with = 0:01 and with a dispersion ofz= 0:09. Using both the spectroscopic and the photometric redshifts, we reached an overall redshift completeness of about 98%. The size of the sample, the redshift complete- ness, the availability of high quality photometric redshifts and multicolor spectral energy distributions make the K20 survey database one of the most complete samples available to date for constraining the currently competing scenarios of galaxy formation and for a variety of other galaxy evolution studies.

The effect of non-gravitational gas heating in groups and clusters of galaxies

We present a detailed study of a set of gas-dynamical simulations of galaxy groups and clusters in a flat, �CDM model with m = 0.3, aimed at exploring the effect of non– gravitational heating on the observable properties of the intracluster medium (ICM). We use GASOLINE, a version of the code PKDGRAV that includes an SPH description of hydrodynamics to simulate the formation of four cosmic halos with virial temperatures in the range 0.5∼ T ∼ 8 keV. These simulations resolve the structure and properties of the intra–cluster medium (ICM) down to a small fraction of the virial radius, Rvir. At our resolution X–ray luminosities, (LX), of runs with gravitational heating only are in good agreement, over almost two orders of magnitude in mass, with analytical predictions, that assume a universal profile for CDM halos. For each simulated structure, non–gravitational heating of the ICM is implemented in two different ways: (1) by imposing a minimum entropy floor, Sfl, at a given redshift, that we take in the range 16 z 65; (2) by gradually heating gas within collapsed regions, proportionally to the supernova rate expected from semi–analytical modeling of galaxy formation in halos having mass equal to that of the simulated systems. Our main results are the following. (a) An extra heating energy Eh∼ 1 keV per gas particle within Rvir at z = 0 is required to reproduce the observed LX–T relation, independent of whether it is provided in an impulsive way to create an entropy floor Sfl = 50–100 keV cm 2 , or is modulated in redshift according to the star formation rate; our SN feedback recipe provides at most Eh ≃ 1/3 keV/part and, therefore, its effect on the LX–T relation is too small to account for the observed LX–T relation. (b) The required heating implies, in small groups with T ∼ 0.5 keV, a baryon fraction as low as ∼ 40% of the cosmic value at Rvir/2; this fraction increases to about 80% for a T ≃ 3 keV cluster. (c) Temperature profiles are almost scale free across the whole explored mass range, with T decreasing by a factor of three at the virial radius. (d) The mass–temperature relation is almost unaffected by non–gravitational heating and follows quite closely the M ∝ T 3/2 scaling; however, when compared with data on the M500–Tew relation, it has a ∼ 40% higher normalization. This discrepancy is independent of the heating scheme adopted. The inclusion of cooling in a run of a small group steepens the central profile of the potential well while removing gas from the diffuse phase. This has the effects of increasing Tew by ∼ 30%, possibly reconciling the simulated and the observed M500–Tew relations, and of decreasing LX by ∼ 40%. However, in spite of the inclusion of SN feedback energy, almost 40% of the gas drops out from the hot diffuse phase, in excess of current observational estimates of the amount of cold baryons in galaxy systems. Likely, only a combination of different heating sources (SNe and AGNs) and cooling will be able to reproduce both the LX–Tew and M500–Tew relations, as observed in groups and clusters, while balancing the cooling runaway.

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

The Luminosity-Temperature Relation for Groups and Clusters of Galaxies

We model the effects of shocks on the diffuse, X-ray-emitting baryons in clusters of galaxies. Shocks separate the infalling from the inner gas nearly at equilibrium and dominate the compression and the density gradients of the latter in the dark matter potential of the cluster. We find that, independently of the detailed shape of the potential, the density gradient is steeper and the compression factor larger for the richer clusters. We show, considering the different merging histories, that in the hierarchical cosmogony the above effects lead, in X-rays, to a luminosity-temperature relation L ∝ T5 at the scale of groups that flattens down to L ∝ T3 for rich clusters in accord with the observations and then saturates toward L ∝ T2 for higher temperatures. From the merging histories, we also compute statistical fluctuations of the L-T correlation.

Cooling and heating the intracluster medium in hydrodynamical simulations

We discuss tree+SPH (smoothed-particle hydrodynamics) simulations of galaxy clusters and groups, aimed at studying the effect of cooling and non-gravitational heating on observable properties of the intracluster medium (ICM). We simulate at high resolution four group- and cluster-sized haloes, with virial masses in the range (0.2‐4) × 10 14 M� , extracted from a cosmological simulation of a flat �-cold dark matter model. We discuss the effects of using different SPH implementations and show that high resolution is mandatory to correctly follow the cooling pattern of the ICM. Our recipes for non-gravitational heating release energy to the gas either in an impulsive way, at some heating redshift, or by modulating the heating as a function of redshift according to the star formation history predicted by a semi-analytic model of galaxy formation. Our simulations demonstrate that cooling and non-gravitational heating exhibit a rather complex interplay in determining the properties of the ICM: results on the amount of star formation and on the X-ray properties are sensitive not only to the amount of heating energy, but also depend on the redshift at which it is assigned to gas particles. All of our heating schemes that correctly reproduce the X-ray scaling properties of clusters and groups do not succeed in reducing the fraction of collapsed gas below a level of 20 (30) per cent at the cluster (group) scale, which appears to be in excess of observational constraints. Finally, gas compression in cooling cluster regions causes an increase of the temperature and a steepening of the temperature profiles, independent of the presence of non-gravitational heating processes. This is inconsistent with recent observational evidence for a decrease of gas temperature towards the centre of relaxed clusters. Provided these discrepancies persist even for a more refined modelling of energy feedback from supernova or active galactic nuclei, they may indicate that some basic physical process is still missing in hydrodynamical simulations.

The Assembly of Massive Galaxies from Near-Infrared Observations of the Hubble Deep Field-South

We use a deep KAB ≤ 25 galaxy sample in the Hubble Deep Field-South to trace the evolution of the cosmological stellar mass density from z 0.5 to z 3. We find clear evidence for a decrease of the average stellar mass density at high redshift, 2 ≤ z ≤ 3.5, that is 15% of the local value, 2 times higher than observed in the Hubble Deep Field-North. To take into account for the selection effects, we define a homogeneous subsample of galaxies with 1010 M☉ ≤ M* ≤ 1011 M☉: in this sample, the mass density at z > 2 is 20% of the local value. In the mass-limited subsample at z > 2, the fraction of passively fading galaxies is at most 25%, although they can contribute up to about 40% of the stellar mass density. On the other hand, star-forming galaxies at z > 2 form stars with an average specific rate of at least 4 × 10-10 yr-1, 3 times higher than the z ≤ 1 value. This implies that UV-bright star-forming galaxies are substancial contributors to the rise of the stellar mass density with cosmic time. Although these results are globally consistent with Λ-CDM scenarios, the present rendition of semianalytic models fails to match the stellar mass density produced by more massive galaxies present at z > 2.