Quiescent Galaxies at $z>2$ from CAPERS spectroscopy and their Number densities

Abstract

Recent JWST observations have revealed an overabundance of quiescent galaxies at high redshift. Photometric methods, such as photometry-derived sSFRs, $UVJ$ colors, and synthetic $ug{i}_s$ colors, are commonly used to identify quiescent galaxies, but their completeness and purity remain poorly constrained. Here we present a spectroscopically selected sample of 19 quiescent galaxies at $2\le z\le 4$ from the CANDELS-Area Prism Epoch of Reionization Survey (CAPERS). They are selected with $M_{\ast }>10^{9.5}{M}_{\odot }$ and a specific star formation rate below 20% of the inverse cosmic age. These galaxies formed 50% of their stellar mass at $z\sim 3.0$--4.6 and quenched rapidly on timescales of $\sim 0.1$--0.2 Gyr. These quenching timescales are comparable to those of other high-redshift QGs but shorter than those at lower redshift. Using the spectroscopic sample as a benchmark, we assess commonly adopted photometric QG selection methods. We find completeness of 63--89% and purities of only 40--63%, with strong emission-line galaxies constituting the dominant source of contamination. Applying completeness and purity corrections calibrated from the spectroscopic sample, we measure the number density of quiescent galaxies at $2\le z\le 5$. The resulting number densities exceed the predictions of most current galaxy-formation simulations at $2<z<4$, with the largest discrepancy at $M_{\ast }\ge 10^{10.5}{M}_{\odot }$. This discrepancy suggests that current cosmological models may not assemble and quench massive galaxies sufficiently early or efficiently.

36 pages, 12 figures