New Technique to Select Recent Fast-Quenching Galaxies at Z ~ 2 Using the Optical Colours

ABSTRACT Many massive quiescent galaxies have been discovered at z > 2 thanks to multiwavelength 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 time-scale (≤0.1 Gyr) at z ∼ 2 photometrically. We focus on a spectral break at ∼1600 Å that appears for such fast-quenching galaxies ∼1 Gyr after quenching when early A-type stars go out, but late A-type stars still live. This spectral break at z ∼ 2 is similar to a Lyman break at z ∼ 4. We construct a set of colour criteria for z ∼ 2 fast-quenching galaxies on g − r versus r − i and i − J versus J − H or $\rm {\it i}-[3.6]$ versus $\rm [3.6]-[4.5]$ colour 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 catalogue. Galaxies with photometric and/or spectroscopic redshifts z ∼ 2 and low specific star formation rates are successfully selected using these colours. The number density of these fast-quenching galaxy candidates at z ∼ 2 suggests that massive galaxies not so far above the star formation main sequence at z = 3–4 should be their progenitors.