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 ($R_{e}-M_{\ast}$) distributions using a
stellar-mass complete sample of $\sim1.5$ million galaxies, covering $\sim100$
deg$^2$, with $\log(M_{\ast}/M_{\odot})>10.2~(9.2)$ 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 $\log(M_{\ast}/M_{\odot})<11$, 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 $R_{e}-M_{\ast}$ relations of
both populations have a form of 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 $\log(M_{p}/M_{\odot})=10.2-10.6$ the relation follows
$R_{e}\propto M_{\ast}^{0.1}$; above $M_{p}$ the relation is steeper and
follows $R_{e}\propto M_{\ast}^{0.6-0.7}$. For star-forming galaxies, below
$\log(M_{p}/M_{\odot})\sim10.7$ the relation follows $R_{e}\propto
M_{\ast}^{0.2}$; above $M_{p}$ the relation evolves with redshift and follows
$R_{e}\propto M_{\ast}^{0.3-0.6}$. 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 process) without significant
structural transformation. Interestingly, the pivot stellar mass of the
$R_{e}-M_{\ast}$ relations coincides with mass at which half of the galaxy
population is quiescent, implied that the pivot mass represents the transition
of galaxy growth from being dominated by in-situ star formation to being
dominated by (dry) mergers.
