Abstract. We present a detailed analysis of the stellar mass content of galaxies up to z = 2.5 as obtained from the K20 spectrophotometric galaxy sample. We have applied and compared two different methods to estimate the stellar mass M * from broad-band photometry: a Maximal Age approach, where we maximize the age of the stellar population to obtain the maximal mass compatible with the observed R − K color, and a Best Fit model, where the best-fitting spectrum to the complete UBVRIzJK s multicolor distribution is used. We find that the M * /L ratio decreases with redshift: in particular, the average M * /L ratio of early type galaxies decreases with z, with a scatter that is indicative of a range of star-formation time-scales and redshift of formation. More important, the typical M * /L ratio of massive early type galaxies is larger than that of less massive ones, suggesting that their stellar population formed at higher z. We show that the final K20 galaxy sample spans a range of stellar masses from M * = 10 9 M to M * = 10 12 M : massive galaxies (M * ≥ 10 11 M ) are common at 0.5 < z < 1, and are detected also up to z 2. We compute the Galaxy Stellar Mass Function at various z, of which we observe only a mild evolution (i.e. by 20-30%) up to z 1. At z > 1, the evolution in the normalization of the GSMF appears to be much faster: at z 2, about 35% of the present day stellar mass in objects with M * 10 11 M appear to have assembled. We also detect a change in the physical nature of the most massive galaxies: at z < ∼ 0.7, all galaxies with M > 10 11 M are early type, while at higher z a population of massive star-forming galaxies progressively appears. We finally analyze our results in the framework of Λ-CDM hierarchical models. First, we show that the large number of massive galaxies detected at high z does not violate any fundamental Λ-CDM constraint based on the number of massive DM halos. Then, we compare our results with the predictions of several renditions of both semianalytic as well as hydro-dynamical models. The predictions from these models range from severe underestimates to slight overestimates of the observed mass density at ≤2. We discuss how the differences among these models are due to the different implementation of the main physical processes.

Abstract. We present the results of VLT optical spectroscopy of a complete sample of 78 EROs with R − Ks ≥ 5 over a field of 52 arcmin 2 . About 70% of the 45 EROs with Ks ≤ 19.2 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 we derive a minimum age of ∼3 Gyr, corresponding to a formation redshift of z f &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 ) &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 ∼ 1.

Abstract. We present the galaxy rest-frame near-IR Luminosity Function (LF) and its cosmic evolution to z ∼ 1.5 based on a spectroscopic survey of a magnitude limited sample of galaxies with K s < 20 (the K20 survey, Cimatti et al. 2002b). The LFs have been derived in the rest-frame J and K s bands. Their evolution is traced using three different redshift bins (z mean 0.5, 1, 1.5) and comparing them to the Local near-IR Luminosity Function. The luminosity functions at different redshifts are fairly well fitted by Schechter functions at z < 1.3. The faint-end of the LFs (L < L * ) is consistent with the local estimates, with no evidence for a change either in the slope or normalization up to z < 1.3. At higher redshift this part of the luminosity function is not well sampled by our data. Viceversa, the density of luminous galaxies (M Ks − 5 log h 70 < −25.5) is higher than locally at all redshifts and relatively constant or mildly increasing with redshift within our sample. The data are consistent with a mild luminosity evolution both in the J-and K s -band up to z 1.5, with an amplitude of about ∆M J −0.69 ± 0.12 and ∆M K −0.54 ± 0.12 at z ∼ 1. Pure density evolution is not consistent with the observed LF at z ≤ 1. Moreover, we find that red and early-type galaxies dominate the bright-end of the LF, and that their number density shows at most a small decrease (<30%) up to z 1, thus suggesting that massive elliptical galaxies were already in place at z 1 and they should have formed their stars and assembled their mass at higher redshift. There appears to be a correlation of the optical/near-IR colors with near-IR luminosities, the most luminous/massive galaxies being red/old, the low-luminous galaxies being instead dominated by blue young stellar populations. We also investigate the evolution of the near-IR comoving luminosity density to z 1.5, finding a slow evolution with redshift (ρ λ (z) = ρ λ (z = 0)(1+z) β(λ) with β(J) 0.70 and β(K s ) 0.37). Finally, we compare the observed LFs with the predictions of a set of the most updated hierarchical merging models. Such a comparison shows that the current versions of hierarchical models overpredict significantly the density of low luminosity galaxies at z ≤ 1 and underpredict the density of luminous galaxies at z ≥ 1, whereas passive evolution models are more consistent with the data up to z ∼ 1.5. The GIF model (Kaufmann et al. 1999) shows a clear deficiency of red luminous galaxies at z ∼ 1 compared to our observations and predicts a decrease of luminous galaxies with redshift not observed in our sample.Key words. galaxies: elliptical and lenticular, evolution, formation, luminosity function -cosmology: observationsinfrared: galaxies IntroductionOver the past few years, a wealth of observations from deep surveys of optically-selected high-redshift galaxies (e.g. Madau et al. 1996;Steidel et al. 1999), complemented by observations in the far-IR/sub-mm (Hughes et al. 1998; Send offprint requests to: L. Pozzetti, e-mail: lucia@bo.astro.it Based on observations ma...

Abstract. The K20 survey is a near infrared-selected, deep (K s < 20) redshift survey targeting galaxies in two independent regions of the sky, the Chandra Deep Field South and the field around the quasar 0055−2659, for a total area of 52 arcmin 2 . The total K s -selected sample includes 545 objects. Low-resolution (R ≈ 300−600) optical spectra for 525 of them have been obtained with the FORS1/FORS2 spectrographs at the ESO/VLT, providing 501 spectroscopic identifications (including 12 type-1 AGN and 45 stars); consequently, we were able to measure redshifts and identify stars in 96% of the observed objects, whereas the spectroscopic completeness with respect to the total photometrically selected sample is 92% (501/545). The K20 survey is therefore the most complete spectroscopic survey of a near infrared-selected sample to date. The K20 survey contains 444 spectroscopically identified galaxies, covering a redshift range of 0.05 < z < 2.73, with a mean redshift z = 0.75; excluding the 32 "low-quality" redshifts does not significantly change these values. This paper describes the final K20 spectroscopic catalogue, along with the technique used to determine redshifts, measure the spectral features and characterize the spectra. The classification of the galaxy spectra has been performed according to a simple parametric recipe that uses the equivalent widths of the two main emission lines ([OII]λ3727 and Hα+ [NII]) and two continuum indices (the 4000 Å break index, D4000, and a near-UV color index, C(28-39)). We defined three main spectroscopic classes: red early-type galaxies, blue emission-line galaxies and the intermediate galaxies, which show emission lines but a red continuum. More than 95% of the examined galaxies is included in one of these spectral types and a composite spectrum is built for each of the three galaxy classes. The full spectroscopic catalogue, the reduced individual spectra and the composite spectra are released to the community through the K20 web page (http://www.arcetri.astro.it/∼k20/). The blue emission-line and the early-type galaxies have been divided in redshift bins, and the corresponding composite spectra have been built, in order to investigate the evolution of the spectral properties of the K20 galaxies with redshift. The early-type average spectra are remarkable in their similarity, showing only subtle but systematic differences in the D4000 index, which are consistent with the ageing of the stellar population. Conversely, the star-forming galaxies present a significant "blueing" of the optical/near-UV continuum with redshift, although the [OII] equivalent width remains constant (∼33 Å) in the same redshift intervals. We reproduce the observed properties with simple, dust-free population synthesis models, suggesting that the highredshift galaxies are younger and more active than those detected at lower redshift, whilst the equivalent width of the emission lines apparently require a lower metallicity for the low-redshift objects. This may be consistent with the metallicity-luminosi...

We present an analysis of the evolution of galaxy clustering in the redshift interval 0 # z # 4:5 in the Hubble Deep Field South (HDF-South). The HST optical data are combined with infrared ISAAC/VLT observations, and photometric redshifts are used for all the galaxies brighter than I AB # 27:5. The clustering signal is obtained in different redshift bins using two different approaches: a standard one, which uses the best redshift estimate of each object, and a second one, which takes into account the redshift probability function of each object. This second method makes it possible to improve the information in the redshift intervals where contamination from objects with insecure redshifts is important. With both methods, we find that the clustering strength up to z . 3:5 in the HDF-South is consistent with the previous results in the HDF-North. Whereas at redshift lower than z , 1 the HDF galaxy population is un/anti-biased ðb # 1Þ with respect to the underlying dark matter, at high redshift the bias increases up to bðz , 3Þ . 2-3, depending on the cosmological model. These results support previous claims that, at high redshift, galaxies are preferentially located in massive haloes, as predicted by the biased galaxy formation scenario. In order to quantify the impact of cosmic errors on our analyses, we have used analytical expressions from Bernstein. Once the behaviour of higher-order moments is assumed, our results show that errors in the clustering measurements in the HDF surveys are indeed dominated by pure shot-noise in most regimes, as assumed in our analysis. We also show that future observations with instruments like the Advanced Camera on HST will improve the signal-to-noise ratio by at least a factor of 2, as a consequence, more detailed analyses of the errors will be required. In fact, pure shot-noise will give a smaller contribution with respect to other sources of errors, such as finite volume effects or non-Poissonian discreteness effects.Key words: galaxies: evolution -galaxies: haloes -galaxies: formation -cosmology: observations -large-scale structure of Universe. I N T R O D U C T I O NIt is well known that the evolution of the dark matter clustering can be reliably used to put strong constraints on cosmological models. In fact, the growth of density fluctuations depends on the main cosmological parameters, namely the contribution of matter and cosmological constant to the present total energy density (V 0m and V 0L , respectively). This result, confirmed by high-resolution N-body simulations (e.g. Jenkins et al. 1998), has been used to build a semi-empirical model which suitably relates the linear perturbation scale to the final non-linear scale of the same perturbation after collapse (Hamilton et al. 1991). This technique can be used to compute analytically the evolved correlation P

We use a deep K AB ≤ 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 ≤ 3.2, that is 15 +25 −5 % of the local value, two times higher than what observed in the Hubble Deep Field North. To take into account for the selection effects, we define a homogeneous subsample of galaxies with 10 10 M ⊙ ≤ M * ≤ 10 11 M ⊙ : in this sample, the mass density at z > 2 is 20 +20 −5 % 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 at least <Ṁ/M * >≃ 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 semi analytic models fails to match the stellar mass density produced by more massive galaxies present at z > 2.

Abstract. The K20 survey is an ESO VLT optical and near-infrared spectroscopic survey aimed at obtaining spectral information and redshifts of a complete sample of about 550 objects to K s ≤ 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 K s -band photometric catalog. Extensive simulations showed that the sample is photometrically highly complete to K s = 20. The observed galaxy counts and the R − K s color distribution are consistent with literature results. We observed spectroscopically 94% of the sample, reaching a spectroscopic redshift identification completeness of 92% to K s ≤ 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 redshifts for the galaxies lacking spectroscopic redshifts. The results show a very good agreement between the spectroscopic and the photometric redshifts with < z spe − z phot > = 0.01 and with a dispersion of σ ∆z = 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 completeness, 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.

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 bulgedominated 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 Sérsic 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.