Star formation and dust obscuration at z~2: galaxies at the dawn of
  downsizing
  We present first results of a study aimed to constrain the star formation
rate and dust content of galaxies at z~2. We use a sample of BzK-selected
star-forming galaxies, drawn from the COSMOS survey, to perform a stacking
analysis of their 1.4 GHz radio continuum as a function of different stellar
population properties, after removing AGN contaminants from the sample. Dust
unbiased star formation rates are derived from radio fluxes assuming the local
radio-IR correlation. The main results of this work are: i) specific star
formation rates are constant over about 1 dex in stellar mass and up to the
highest stellar mass probed; ii) the dust attenuation is a strong function of
galaxy stellar mass with more massive galaxies being more obscured than lower
mass objects; iii) a single value of the UV extinction applied to all galaxies
would lead to grossly underestimate the SFR in massive galaxies; iv) correcting
the observed UV luminosities for dust attenuation based on the Calzetti recipe
provide results in very good agreement with the radio derived ones; v) the mean
specific star formation rate of our sample steadily decreases by a factor of ~4
with decreasing redshift from z=2.3 to 1.4 and a factor of ~40 down the local
Universe.
  These empirical SFRs would cause galaxies to dramatically overgrow in mass if
maintained all the way to low redshifts, we suggest that this does not happen
because star formation is progressively quenched, likely starting from the most
massive galaxies.
