New technique to select recent fast-quenching galaxies at $z\sim 2$ using the optical colors

Abstract

Many massive quiescent galaxies have been discovered at $z>2$ thanks to multi-wavelength deep and wide surveys, however, substantial deep near-infrared spectroscopic observations are needed to constrain their star-formation histories statistically. Here, we present a new technique to select quiescent galaxies with a short quenching timescale ($\le 0.1$ Gyr) at $z\sim 2$ photometrically. We focus on a spectral break at $\sim 1600$ Å~that appears for such fast-quenching galaxies $\sim 1$ Gyr after quenching when early A-type stars go out, but late A-type stars still live. This spectral break at $z\sim 2$ is similar to a Lyman break at $z\sim 4$. We construct a set of color criteria for $z\sim 2$ fast-quenching galaxies on $g-r$ vs. $r-i$ and $i-J$ vs. $J-H$ or $\mathrm{i}-[3.6]$ vs. $[3.6]-[4.5]$ color diagrams, which are available with the existing and/or future wide imaging surveys, by simulating various model galaxy spectra and test their robustnesses using the COSMOS2020 catalog. Galaxies with photometric and/or spectroscopic redshifts $z\sim 2$ and low specific star formation rates are successfully selected using these colors. The number density of these fast-quenching galaxy candidates at $z\sim 2$ suggests that massive galaxies not so far above the star-formation main sequence at $z=3-4$ should be their progenitors.

12 pages, 11 figures, accepted for publication in MNRAS