MOIRCS Deep Survey. X. Evolution of Quiescent Galaxies as a Function of
  Stellar Mass at 0.5<z<2.5
  We study the evolution of quiescent galaxies at 0.5<z<2.5 as a function of
stellar mass, using very deep NIR imaging data taken with the Multi-Object
Infrared Camera and Spectrograph on the Subaru Telescope in the GOODS-North
region. The deep NIR data allow us to construct a stellar mass-limited sample
of quiescent galaxies down to ~10^{10} Msun even at z~2 for the first time. We
selected quiescent galaxies with age/tau>6 by performing SED fitting of the
multi broad-band photometry from the U to Spitzer 5.8um bands with the
population synthesis model of Bruzual & Charlot (2003) where exponentially
decaying star formation histories are assumed. The number density of quiescent
galaxies increases by a factor of ~3 from 1.0<z<1.5 to 0.5<z<1.0, and by a
factor of ~10 from 1.5<z<2.5 to 0.5<z<1.0, while that of star-forming galaxies
with age/tau<4 increases only by factors of ~2 and ~3 in the same redshift
ranges. At 0.5<z<2.5, the low-mass slope of the stellar mass function of
quiescent galaxies is alpha ~ 0 -- 0.6, which is significantly flatter than
those of star-forming galaxies (alpha ~ -1.3 -- -1.5). As a result, the
fraction of quiescent galaxies in the overall galaxy population increases with
stellar mass in the redshift range. The fraction of quiescent galaxies at
10^{11}-10^{11.5} Msun increases from ~20-30% at z~2 to ~40-60% at z~0.75,
while that at 10^{10}-10^{10.5} Msun increases from <~ 5% to ~15% in the same
redshift range. These results could suggest that the quenching of star
formation had been more effective in more massive galaxies at 1<~z<~2. Such a
mass-dependent quenching could explain the rapid increase of the number density
of ~M* galaxies relative to lower-mass galaxies at z >~ 1-1.5.
