Slow Evolution of the Specific Star Formation Rate at z>2: The Impact of
  Dust, Emission Lines, and A Rising Star Formation History
  We measure the evolution of the specific star formation rate (sSFR = SFR /
Mstellar) between redshift 4 and 6 to investigate the previous reports of
"constant" sSFR at z>2. We obtain photometry on a large sample of galaxies at
z~4-6 located in the GOODS-S field that have high quality imaging from HST and
Spitzer. We have derived stellar masses and star formation rates (SFRs) through
stellar population modeling of their spectral energy distributions (SEDs). We
estimate the dust extinction from the observed UV colors. In the SED fitting
process we have studied the effects of assuming a star formation history (SFH)
both with constant SFR and one where the SFR rises exponentially with time. The
latter SFH is chosen to match the observed evolution of the UV luminosity
function. We find that neither the mean SFRs nor the mean stellar masses change
significantly when the rising SFR (RSF) model is assumed instead of the
constant SFR model. When focusing on galaxies with Mstar ~ 5x10^9 Msun, we find
that the sSFR evolves weakly with redshift (sSFR(z) \propto (1+z)^(0.6+/-0.1)
Gyr^-1), consistent with previous results and with recent estimates of the sSFR
at z~2-3 using similar assumptions. We have also investigated the impact of
optical emission lines on our results. We estimate that the contribution of
emission lines to the rest-frame optical fluxes is only modest at z~4 and 5 but
it could reach ~50% at z~6. When emission lines of this strength are taken into
account, the sSFR shows somewhat higher values at high redshifts, according to
the relation sSFR(z) \propto (1+z)^(1.0+/-0.1) Gyr^-1, i.e., ~2.3x higher at
z~6 than at z~2. However, the observed evolution is substantially weaker than
that found at z<2 or that expected from current models (which corresponds to
sSFR(z) \propto (1+z)^(2.5) Gyr^-1). -abridged-
