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

Recent JWST observations have revealed a large population of quiescent galaxies (QGs) at high redshift (z ∼ 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 important for refining the QG sample: it helps to mitigate contamination from dusty star-forming galaxies in the high-mass regime at z ∼ 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⋆/M⊙)>10.6 ) declines rapidly, falling from n ≈ 1.32 × 10−5 Mpc−3 at z ∼ 3–4 to n ∼ 1 × 10−6 Mpc−3 at z ∼ 6, and becomes negligible at z > 6. In contrast, low-mass QGs ( 9.5<log(M⋆/M⊙)<10.6 ) exhibit a constant number density of n ∼ 2 × 10−6 Mpc−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.