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 similar to 2, selected in the z(850)-band from the GOODS-MUSIC catalog. By utilizing the rest frame 8 mu m 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 physical parameters of galaxies. 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 the 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-circle dot yr(-1), the masses from 10(9) to 4 x 10(11) M-circle dot, and 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 have formed within a few tens of Myr and already been copious at <= 100 Myr. In addition, we find that low luminosity 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.