MOIRCS Deep Survey. X. Evolution of quiescent galaxies as a function of stellar mass at 0.5 < z < 2.5

We studied the evolution of quiescent galaxies at 0.5 < z < 2.5 as a function of the stellar mass, using very deep NIR imaging data from the MOIRCS Deep Survey in the GOODS-North region. The deep NIR data allowed us to construct a stellar mass-limited sample of quiescent galaxies down to ∼10 ¹⁰M_⊙, even at z ∼ 2, for the first time. We selected quiescent galaxies with age /τ > 6 by performing a SED fitting of the multi broad-band photometry from the U to Spitzer 5.8-m bands with the population synthesis model of Bruzual and Charlot (2003, MNRAS, 344, 1000), 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 /τ < 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 α ∼ 0-0.6, which is significantly flatter than those of star-forming galaxies (α ∼ -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 10¹¹-10^11.5M_⊙ increases from ∼ 20%-30% at z ∼ 2 to ∼ 40%-60% at z ∼ 0.75, while that at 10¹⁰-10^10.5M_⊙ 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 massdependent quenching could explain the rapid increase in the number density of ≲ M* galaxies relative to lower-mass galaxies at z ≳ 1-1.5.