A combined VANDELS and LEGA-C study: the evolution of quiescent galaxy
  size, stellar mass and age from $\mathbf{\textit{z} = 0.6}$ to
  $\mathbf{\textit{z} = 1.3}$
  We study the relationships between stellar mass, size and age within the
quiescent population, using two mass-complete spectroscopic samples with
$\mathrm{log_{10}}(M_{\star}/\mathrm{M_{\odot}})>10.3$, taken from VANDELS at
$1.0<z<1.3$, and LEGA-C at $0.6<z<0.8$. Using robust D$_{n}$4000 values, we
demonstrate that the well-known 'downsizing' signature is already in place by
$z\simeq1.1$, with D$_{n}$4000 increasing by $\simeq0.1$ across a $\simeq$ 1
dex mass interval for both VANDELS and LEGA-C. We then proceed to investigate
the evolution of the quiescent galaxy stellar mass-size relation from
$z\simeq1.1$ to $z\simeq0.7$. We find the median size increases by a factor of
$1.9\pm{0.1}$ at $\mathrm{log_{10}}(M_{\star}/\mathrm{M_{\odot}})=10.5$, and
see tentative evidence for flattening of the relation, finding slopes of
$\alpha=0.72\pm0.06$ and $\alpha=$ $0.56\pm0.04$ for VANDELS and LEGA-C
respectively. We finally split our sample into galaxies above and below our
fitted mass-size relations, to investigate how size and D$_{n}$4000 correlate.
For LEGA-C, we see a clear difference, with larger galaxies found to have
smaller D$_{n}$4000 at fixed stellar mass. Due to the faintness and smaller
numbers of the VANDELS sample, we cannot confirm whether a similar relation
exists at $z\simeq1.1$. We consider whether differences in stellar age or
metallicity are most likely to drive this size-D$_{n}$4000 relation, finding
that any metallicity differences are unlikely to fully explain the observed
offset, meaning smaller galaxies must be older than their larger counterparts.
We find the observed evolution in size, mass and D$_{n}$4000 across the
$\simeq2$ Gyr from $z\sim1.1$ to $z\sim0.7$ can be explained by a simple toy
model in which VANDELS galaxies evolve passively, whilst experiencing a series
of minor mergers.
