The K20 survey

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 UBVRIzJKs 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*=109 Msun to M*=1012 Msun: massive galaxies (M*≥1011 Msun) are common at 0.5 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* ≃ 1011 Msun 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>1011 Msun 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. Based on observations made at the European Southern Observatory, Paranal, Chile (ESO LP 164.O-0560). Appendices A and B are only available in electronic form at http://www.edpsciences.org