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

Recent JWST observations have revealed an overabundance of quiescent galaxies at high redshift. Photometric methods, such as photometry-derived sSFRs, UVJ colors, and synthetic ugi 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 at most z at most 4 from the CANDELS-Area Prism Epoch of Reionization Survey (CAPERS). They are selected with M > 10 9.5 M and a specific star formation rate below 20% of the inverse cosmic age. These galaxies formed 50% of their stellar mass at z about 3.0 --4.6 and quenched rapidly on timescales of about0.1 --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 at most z at most 5. The resulting number densities exceed the predictions of most current galaxy-formation simulations at 2<z<4, with the largest discrepancy at M at least 10 10.5 M. This discrepancy suggests that current cosmological models may not assemble and quench massive galaxies sufficiently early or efficiently.