Star formation rates and masses of z ~ 2 galaxies from multicolour photometry

Abstract

Fitting synthetic spectral energy distributions (SED) to the multi-band photometry of galaxies to derive their star formation rates (SFR), stellar masses, ages, etc. requires making a priori assumptions about their star formation histories (SFH). A widely adopted parameterization of the SFH, the so-called tau-models where SFR goes as e^{-t/tau) is shown to lead to unrealistically low ages when applied to star forming galaxies at z ~ 2, a problem shared by other SFHs when the age is left as a free parameter in the fitting. This happens because the SED of such galaxies, at all wavelengths, is dominated by their youngest stellar populations, which outshine the older ones. Thus, the SED of such galaxies conveys little information on the beginning of star formation. To cope with this problem, we explore a variety of SFHs, such as constant SFR and inverted-tau models - with SFR as e^{+t/tau) - along with various priors on age, including assuming that star formation started at high redshift in all the galaxies. We find that inverted-tau models with such latter assumption give SFRs and extinctions in excellent agreement with the values derived using only the UV part of the SED. These models are also shown to accurately recover the SFRs and masses of mock galaxies at z ~ 2 constructed from semi-analytic models. All other explored SFH templates do not fulfil these two test. In particular, direct-tau models with unconstrained age in the fitting procedure overstimate SFRs and underestimate stellar mass, and would exacerbate an apparent mismatch between the cosmic evolution of the volume densities of SFR and stellar mass. We conclude that for high-redshift star forming galaxies an exponentially increasing SFR with a high formation redshift is preferable to other forms of the SFH so far adopted in the literature.