HST/WFC3 grism observations of $z\sim1$ clusters: evidence for evolution
  in the mass-size relation of quiescent galaxies from poststarburst galaxies
  Minor mergers have been proposed as the driving mechanism for the size growth
of quiescent galaxies with decreasing redshift. The process whereby large
star-forming galaxies quench and join the quiescent population at the large
size end has also been suggested as an explanation for this size growth. Given
the clear association of quenching with clusters, we explore this mechanism by
studying the structural properties of 23 spectroscopically identified recently
quenched (or "poststarburst" (PSB)) cluster galaxies at $z\sim1$. Despite clear
PSB spectral signatures implying rapid and violent quenching, 87\% of these
galaxies have symmetric, undisturbed morphologies in the stellar continuum.
Remarkably, they follow a mass-size relation lying midway between the
star-forming and quiescent field relations, with sizes $0.1$ dex smaller than
$z\sim1$ star-forming galaxies at log$(M_{*}/M_{\odot})=10.5$. This implies a
rapid change in the light profile without directly effecting the stellar
distribution, suggesting changes in the mass-to-light ratio gradients across
the galaxy are responsible. We develop fading toy models to explore how
star-forming galaxies move across the mass-size plane as their stellar
populations fade to match those of the PSBs. "Outside-in" fading has the
potential to reproduce the contraction in size and increase in bulge-dominance
observed between star-forming and PSB cluster galaxies. Since cluster PSBs lie
on the large size end of the quiescent mass-size relation, and our previous
work shows cluster galaxies are smaller than field galaxies, the sizes of
quiescent galaxies must grow both from the quenching of star-forming galaxies
and dry minor mergers.
