  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.
