Star formation and mass assembly in high redshift galaxies
  We study the star formation and the mass assembly process of 0.3<=z<2.5
galaxies using their IR emission from MIPS 24um band. We used an updated
version of the GOODS-MUSIC catalog, extended by the addition of mid-IR fluxes.
We compared two different estimators of the Star Formation Rate: the total
infrared emission derived from 24um, estimated using both synthetic and
empirical IR templates, and the multiwavelength fit to the full galaxy SED. For
both estimates, we computed the SFR Density and the Specific SFR. The two SFR
tracers are roughly consistent, given the uncertainties involved. However, they
show a systematic trend, IR-based estimates exceeding the fit-based ones as the
SFR increases. We show that: a) at z>0.3, the SFR is well correlated with
stellar mass, and this relationship seems to steepen with redshift (using
IR-based SFRs); 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; d) at z~2, massive
galaxies are actively star-forming, with a median SFR 300 Msun/yr. During this
epoch, they assemble a substantial part of their final stellar mass; e) the
SSFR shows a clear bimodal distribution. The analysis of the SFRD and the SSFR
seems to support the downsizing scenario, according to which high mass galaxies
have formed their stars earlier and faster than their low mass counterparts. A
comparison with theoretical models indicates that they follow the global
increase in the SSFR with redshift and predict the existence of quiescent
galaxies even at z>1.5, but they systematically underpredict the average SSFR.
