2021ApJ...921...38K

We present the galaxy size-mass (R_e-M_*) distributions using a stellar mass complete sample of ∼1.5 million galaxies, covering ∼100 deg², with log(M_*/M_☉)> 10.2(9.2) over the redshift range 0.2 < z < 1.0 (z < 0.6) from the second public data release of the Hyper Suprime-Cam Subaru Strategic Program. We confirm that, at fixed redshift and stellar mass over the range of log(M_*/M_☉)< 11, star-forming galaxies are on average larger than quiescent galaxies. The large sample of galaxies with accurate size measurements, thanks to the excellent imaging quality, also enables us to demonstrate that the R_e-M_* relations of both populations have a form of a broken power law, with a clear change of slopes at a pivot stellar mass M_p. For quiescent galaxies, below an (evolving) pivot mass of log(M_p/M_☉)=10.2–10.6, the relation follows R_e∝M_*^0.1; above M_p the relation is steeper and follows R_e∝M_*^0.6–0.7. For star-forming galaxies, below log(M_p/M_☉)∼10.7 the relation follows R_e∝M_*^0.2; above M_p the relation evolves with redshift and follows R_e∝M_*^0.3–0.6. The shallow power-law slope for quiescent galaxies below M_p indicates that large low-mass quiescent galaxies have sizes similar to those of their counterpart star-forming galaxies. We take this as evidence that large low-mass quiescent galaxies have been recently quenched (presumably through environment-specific processes) without significant structural transformation. Interestingly, the pivot stellar mass of the R_e-M_* relations for both populations also coincides with the mass at which half of the galaxy population is quiescent, implying that the pivot mass represents the transition of galaxy growth from being dominated by in situ star formation to being dominated by (dry) mergers.