MOIRCS Deep Survey. VIII. Evolution of Star Formation Activity as a
  Function of Stellar Mass in Galaxies since z~3
  We study the evolution of star formation activity of galaxies at 0.5<z<3.5 as
a function of stellar mass, using very deep NIR data taken with Multi-Object
Infrared Camera and Spectrograph (MOIRCS) on the Subaru telescope in the
GOODS-North region. The NIR imaging data reach K ~ 23-24 Vega magnitude and
they allow us to construct a nearly stellar mass-limited sample down to ~
10^{9.5-10} Msun even at z~3. We estimated star formation rates (SFRs) of the
sample with two indicators, namely, the Spitzer/MIPS 24um flux and the
rest-frame 2800A luminosity. The SFR distribution at a fixed Mstar shifts to
higher values with increasing redshift at 0.5<z<3.5. More massive galaxies show
stronger evolution of SFR at z>~1. We found galaxies at 2.5<z<3.5 show a
bimodality in their SSFR distribution, which can be divided into two
populations by a constant SSFR of ~2 Gyr^{-1}. Galaxies in the low-SSFR group
have SSFRs of ~ 0.5-1.0 Gyr^{-1}, while the high-SSFR population shows ~10
Gyr^{-1}. The cosmic SFRD is dominated by galaxies with Mstar = 10^{10-11} Msun
at 0.5<z<3.5, while the contribution of massive galaxies with Mstar =
10^{11-11.5} Msun shows a strong evolution at z>1 and becomes significant at
z~3, especially in the case with the SFR based on MIPS 24um. In galaxies with
Mstar = 10^{10-11.5} Msun, those with a relatively narrow range of SSFR (<~1
dex) dominates the cosmic SFRD at 0.5<z<3.5. The SSFR of galaxies which
dominate the SFRD systematically increases with redshift. At 2.5<z<3.5, the
high-SSFR population, which is relatively small in number, dominates the SFRD.
Major star formation in the universe at higher redshift seems to be associated
with a more rapid growth of stellar mass of galaxies.
