P. Saracco

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.

Extremely compact massive galaxies at z ∼ 1.4

The optical rest-frame sizes of 10 of the most massive (∼5 × 10 11 h −2 M� ) galaxies found in the near-infrared MUNICS survey at 1.2 < z < 1.7 are analysed. Sizes are estimated in both the J and Kfilters. These massive galaxies are at least a factor of 4 +1.9 (±1σ ) smaller in the rest-frame V-band than local counterparts of the same stellar mass. Consequently, the stellar mass density of these objects is (at least) 60 times larger than that of massive ellipticals today. Although the stellar populations of these objects are passively fading, their structural properties are rapidly changing since that redshift. This observational fact disagrees with a scenario where the more massive and passive galaxies are fully assembled at z ∼ 1.4 (i.e. a monolithic scenario) and points towards a dry merger scenario as the responsible mechanism for the subsequent evolution of these galaxies.

The Kormendy relation of massive elliptical galaxies at z∼ 1.5: evidence for size evolution

We present the morphological analysis based on HST-NIC2 (0.075 arcsec pixel -1 ) images in the F160W filter of a sample of nine massive field (>10 11 M ⊙ ) galaxies spectroscopically classified as early-types at 1.2 < z < 1.7. Our analysis shows that all of them are bulge-dominated systems. In particular, six of them are well fitted by a de Vaucouleurs profile (n = 4) suggesting that they can be considered pure elliptical galaxies. The remaining three galaxies are better fitted by a Sersic profile with index 1.9 < n fit < 2.3 suggesting that a disc-like component could contribute up to 30 per cent to the total light of these galaxies. We derived the effective radius R e and the mean surface brightness (SB) (μ) e within R e of our galaxies and we compared them with those of early-types at lower redshifts. We find that the SB (μ) e of our galaxies should get fainter by 2.5 mag from z ∼ 1.5 to ∼0 to match the SB of the local ellipticals with comparable R e , that is, the local Kormendy relation. Luminosity evolution without morphological changes can only explain half of this effect, as the maximum dimming expected for an elliptical galaxy is ∼ 1.6 mag in this redshift range. Thus, other parameters, possibly structural, may undergo evolution and play an important role in reconciling models and observations. Hypothesizing an evolution of the effective radius of galaxies we find that R e should increase by a factor of 1.5 from z ∼ 1.5 to ∼0.

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.

Stellar mass estimates in early-type galaxies: procedures, uncertainties and models dependence

The aim of the present paper is to quantify the dependence of the estimates of luminosities and stellar mass content of early-type galaxies on the different models and model parameters which can be used to analyse the observational data. The paper is organized in two parts. The first one analyses the dependence of the M/C ratios and of the k-corrections in different bands on model parameters (initial mass function, metallicity, star formation history, age), assuming some among the most popular spectrophotometric codes usually adopted to study the evolutionary status of galaxies: Bruzual & Charlot (BC03), Charlot & Bruzual (CB08), Maraston (Ma05), Fioc & Rocca-Volmerange (PEGASE), Silva et al. (GRASIL). The second part of our work is dedicated to quantify the reliability and systematics affecting the mass and luminosity estimates obtained by means of the best-fitting technique applied to the photometric spectral energy distributions (SEDs) of early-type galaxies at 1 < z < 2. To this end, we apply the best-fitting technique to some mock catalogues built on the basis of a wide set of models of early-type galaxies. We then compare the luminosity and the stellar mass estimated from the SED fitting with the true known input values. The goodness of the mass estimate is found to be dependent on the mass estimator adopted to derive it, but masses cannot anyhow be retrieved better than within a factor of 2-3, depending on the quality of the available photometric data and/or on the distance of the galaxies since more distant galaxies are fainter on average and thus affected by larger photometric errors. Finally, we present a new empirical mass estimator based on the K-band apparent magnitude and on the observed (V - K) colour. We show that the reliability of the stellar mass content derived with this new estimator for early-type galaxies and its stability are even higher than those achievable with the best classic estimators, with the not negligible advantage that it does not need any multiwavelength data fitting.

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.

Photometric Redshifts and Selection of High-Redshift Galaxies in the NTT and Hubble Deep Fields*

We present and compare in this paper new photometric redshift catalogs of the galaxies in three public fields: the NTT Deep Field, the HDF-N, and the HDF-S. In the case of the NTT Deep Field, we present here a new photometric catalog, obtained by combining the existing BVrI and JKs with new deep U observations acquired with NTT-SUSI2, and which includes also the contiguous field centered on the zem = 4.7 quasar BR 1202-07. Photometric redshifts have been obtained for the whole sample (NTTDF + HDF-N + HDF-S), by adopting a χ2 minimization technique on a spectral library drawn from the Bruzual and Charlot synthesis models, with the addition of dust and intergalactic absorption. The accuracy, determined from 125 galaxies with known spectroscopic redshifts, is σz ~ 0.08 (0.3) in the redshift intervals z = 0–1.5 (1.5–3.5). The global redshift distribution of I-selected galaxies shows a distinct peak at intermediate redshifts, z 0.6 at IAB ≤ 26 and z 0.8 at IAB ≤ 27.5 followed by a tail extending to z 6. Systematic differences exist among the fields, most notably the HDF-S which contains a much smaller number of galaxies at z 0.9 and at z ≥ 4.5 than the HDF-N. We also present for the first time the redshift distribution of the total IR-selected sample to faint limits (Ks ≤ 21 and J ≤ 22). It is found that the number density of galaxies at 1.25 5 candidates in the HDF filter set and that the four brightest candidates at z > 5 in the HDF-S are indeed most likely M stars.

The population of early-type galaxies at 1 < z < 2 - new clues on their formation and evolution

We present the morphological analysis based on Hubble Space Telescope HST-NICMOS (Near-Infrared Camera and Multi-Object Spectrometer) observations in the F160W filter (λ � 1.6 μm) of a sample of 32 early-type galaxies (ETGs) at 1 5 × 10 11 M� ) ETGs. Thus, major merging cannot fit this requirement. Satellite merging, close encounters and interactions can help at least qualitatively in solving this problem. The younger population of ETGs can be formed later through subsequent episodes of merging which increased progressively their size and assembled their mass down to z ∼ 2. At z < 2, they evolve purely in luminosity since episodes of major merging would bring them far from the local scaling relations.

The density of very massive evolved galaxies to z ≃ 1.7

We spectroscopically identified seven massive, evolved galaxies with magnitudes 17.8 2 supporting a high efficiency in the accretion of the stellar mass in massive haloes in the early Universe.

The Evolution of the Galaxy Luminosity Function in the Rest-Frame Blue Band up to z = 3.5*

We present an estimate of the cosmological evolution of the field galaxy luminosity function (LF) in the rest-frame 4400 A B band up to redshift z = 3.5. To this purpose, we use a composite sample of 1541 I-selected galaxies selected down to IAB = 27.2 and 138 galaxies selected down to KAB = 25 from ground-based and HST multicolor surveys, most notably the new deep JHK images in the Hubble Deep Field-South (HDF-S) taken with the ISAAC instrument at the ESO-VLT telescope. About 21% of the sample has spectroscopic redshifts, and the remaining fraction well-calibrated photometric redshifts. The resulting blue LF shows little density evolution at the faint end with respect to the local values, while at the bright end [MB(AB) < -20] a brightening increasing with redshift is apparent with respect to the local LF. Hierarchical CDM models overpredict the number of faint galaxies by a factor ~3 at z 1. At the bright end the predicted LFs are in reasonable agreement only at low and intermediate redshifts (z 1) but fail to reproduce the pronounced brightening observed in the high-redshift (z ~ 2-3) LF. This brightening could mark the epoch at which major star formation activity is present in the galaxy evolution.