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...

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.

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