Hyper Suprime-Cam Subaru Strategic Program: A Mass-dependent Slope of the Galaxy Size-Mass Relation at z < 1

We present the galaxy size-mass (R-e-M*) distributions using a stellar mass complete sample of similar to 1.5 million galaxies, covering similar to 100 deg(2), with log(M*/M-circle dot) > 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-circle dot) < 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-circle dot) = 10.2-10.6, the relation follows R-e proportional to M-*(0.1); above M-p the relation is steeper and follows R-e proportional to M-*(0.6-0.7). For star-forming galaxies, below log(M-p/M-circle dot) similar to 10.7 the relation follows R-e proportional to M-*(0.2); above M-p the relation evolves with redshift and follows R-e proportional to 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.