A census of quiescent galaxies across $0.5<z<8$ with JWST/MIRI: Mass-dependent number density evolution of quiescent galaxies in the early Universe

Abstract

Recent JWST observations have revealed a large population of quiescent galaxies (QGs) at high redshift ($z\sim 4-8$), challenging current models of early galaxy formation and quenching. Accurate number density estimates are crucial but remain uncertain. We present a systematic study of QGs at $0.5<z<8$ using a mass-complete sample from the JWST/PRIMER survey with deep NIRCam and MIRI imaging. We demonstrate that MIRI photometry is essential for refining the QG sample: it helps to mitigate contamination from dusty star-forming galaxies in the high-mass regime at $z\sim 3-5$ and aids in recovering lower-mass QG candidates at $z>5$ that are often missed without including MIRI data. We find that the evolution of the QG number density is strongly mass-dependent. The density of massive QGs ($\log (M_{\star }/M_{\odot })>10.6$) declines rapidly, falling from $n\approx 1.32\times 10^{-5}\mathrm{Mp}{\mathrm{c}}^{-3}$ at $z\sim 3-4$ to $n<1\times 10^{-6}\mathrm{Mp}{\mathrm{c}}^{-3}$ at $z\sim 6$, and becomes negligible at $z>6$. In contrast, low-mass QGs ($9.5<\log (M_{\star }/M_{\odot })<10.6$) exhibit a remarkably constant number density of $n\sim 3\times 10^{-6}\mathrm{Mp}{\mathrm{c}}^{-3}$ across the redshift range $z=4-8$. This plateau suggests that these high-redshift, low-mass QGs may be galaxies undergoing temporary quenching episodes, likely subject to rejuvenation upon future gas accretion. Comparisons with leading galaxy formation models reveal significant tensions: most models underestimate the abundance of massive QGs at $z>4$ and fail to reproduce the flat density evolution observed for the low-mass population.

published by ApJL, updated to match the published version