The Star Formation History of Redshift z~2 Galaxies: the Role of the Infrared Prior

We build a sample of 298 spectroscopically-confirmed galaxies at redshift z~2, selected in the z_850-band from the GOODS-MUSIC catalog. By exploiting the rest frame 8 mu luminosity as a proxy of the star formation rate (SFR) we check the accuracy of the standard SED-fitting technique, finding it is not accurate enough to provide reliable estimates of the galaxy physical parameters. We then develop a new SED-fitting method that includes the IR luminosity as a prior and a generalized Calzetti law with a variable R_V. Then we exploit such a new method to re-analyze our galaxy sample, and to robustly determine SFRs, stellar masses and ages. We find that there is a general trend of increasing attenuation with the SFR. Moreover, we find that the SFRs range between a few to 10^3 M_sun/yr, the masses from 10^9 to 4 10^11 M_sun, while the ages from a few tens of Myr to more than 1 Gyr. We discuss how individual age measurements of highly attenuated objects indicate that dust must form within a few tens of Myr and be copious already at 100 Myr. In addition, we find that low luminous galaxies harbor, on average, significantly older stellar populations and are also less massive than brighter ones; we discuss how these findings and the well known ‘downsizing’ scenario are consistent in a framework where less massive galaxies form first, but their star formation lasts longer. Finally, we find that the near-IR attenuation is not scarce for luminous objects, contrary to what is customarily assumed; we discuss how this affects the interpretation of the observed M_star/L ratios.