MOIRCS Deep Survey. X. Evolution of Quiescent Galaxies as a Function of Stellar Mass at 0.5 z 2.5

Abstract

Abstract We studied the evolution of quiescent galaxies at 0.5 $\lt$z$\lt$ 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 $\sim\ $10$^{10}\ M_{\odot}$, even at z$\sim\ $ 2, for the first time. We selected quiescent galaxies with age / $\tau$$\gt$ 6 by performing a SED fitting of the multi broad-band photometry from the $U$ to Spitzer 5.8$\ \mu$m bands with the population synthesis model of Bruzual and Charlot (2003, MNRAS, 344, 1000), where exponentially decaying star-formation histories are assumed. The number density of quiescent galaxies increases by a factor of $\sim\ $3 from 1.0 $\lt$z$\lt$ 1.5 to 0.5 $\lt$z$\lt$ 1.0, and by a factor of $\sim\ $10 from 1.5 $\lt$z$\lt$ 2.5 to 0.5 $\lt$z$\lt$ 1.0, while that of star-forming galaxies with age / $\tau$$\lt$ 4 increases only by factors of $\sim\ $2 and $\sim\ $3 in the same redshift ranges. At 0.5 $\lt$z$\lt$ 2.5, the low-mass slope of the stellar mass function of quiescent galaxies is $\alpha$$\sim\ $ 0–0.6, which is significantly flatter than those of star-forming galaxies ($\alpha$$\sim\ $$-$1.3–$-$1.5). As a result, the fraction of quiescent galaxies in the overall galaxy population increases with stellar mass in the redshift range. The fraction of quiescent galaxies at 10$^{11}$–10$^{11.5}\ M_{\odot}$ increases from $\sim\ $20%–30% at z$\sim\ $ 2 to $\sim\ $40%–60% at z$\sim\ $ 0.75, while that at 10$^{10}$–10$^{10.5}\ M_{\odot}$ increases from $\lesssim\ $5% to $\sim\ $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 $\lesssim\ $z$\lesssim\ $ 2. Such a mass-dependent quenching could explain the rapid increase in the number density of $\sim\ M^*$ galaxies relative to lower-mass galaxies at z$\gtrsim$ 1–1.5.