New technique to select recent fast-quenching galaxies at z 2 using the optical colours

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 similar to 2 photometrically. We focus on a spectral break at similar to 1600 angstrom that appears for such fast-quenching galaxies similar to 1 Gyr after quenching when early A-type stars go out, but late A-type stars still live. This spectral break at z similar to 2 is similar to a Lyman break at z similar to 4. We construct a set of colour criteria for z similar to 2 fast-quenching galaxies on g - r versus r - i and i - J versus J - H or i - [3.6] versus [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 similar to 2 and low specific star formation rates are successfully selected using these colours. The number density of these fast-quenching galaxy candidates at z similar to 2 suggests that massive galaxies not so far above the star formation main sequence at z = 3-4 should be their progenitors.