The LEGA-C and SAMI Galaxy Surveys: Quiescent Stellar Populations and
  the Mass-Size Plane across 6 Gyr
  We investigate the change in mean stellar population age and metallicity
([Z/H]) scaling relations for quiescent galaxies from intermediate redshift
($0.60\leq z\leq0.76$) using the LEGA-C Survey, to low redshift ($0.014\leq
z\leq0.10$) using the SAMI Galaxy Survey. We find that, similarly to their
low-redshift counterparts, the stellar metallicity of quiescent galaxies at
$0.60\leq z\leq 0.76$ closely correlates with $M_*/R_\mathrm{e}$ (a proxy for
the gravitational potential or escape velocity), in that galaxies with deeper
potential wells are more metal-rich. This supports the hypothesis that the
relation arises due to the gravitational potential regulating the retention of
metals, by determining the escape velocity required by metal-rich stellar and
supernova ejecta to escape the system and avoid being recycled into later
stellar generations. On the other hand, we find no correlation between stellar
age and $M_*/R_\mathrm{e}^2$ (stellar mass surface density $\Sigma$) in the
LEGA-C sample, despite this being a strong relation at low redshift. We
consider this change in the age--$\Sigma$ relation in the context of the
redshift evolution of the star-forming and quiescent populations in the
mass--size plane, and find our results can be explained as a consequence of
galaxies forming more compactly at higher redshifts, and remaining compact
throughout their evolution. Furthermore, galaxies appear to quench at a
characteristic surface density that decreases with decreasing redshift. The
$z\sim 0$ age--$\Sigma$ relation is therefore a result of building up the
quiescent and star-forming populations with galaxies that formed at a range of
redshifts and so a range of surface densities.
