Sizes, Colour gradients and Resolved Stellar Mass Distributions for the
  Massive Cluster Galaxies in XMMUJ2235-2557 at z = 1.39
  We analyse the sizes, colour gradients, and resolved stellar mass
distributions for 36 massive and passive galaxies in the cluster XMMUJ2235-2557
at z=1.39 using optical and near-infrared Hubble Space Telescope imaging. We
derive light-weighted S\'ersic fits in five HST bands
($i_{775},z_{850},Y_{105},J_{125},H_{160}$), and find that the size decreases
by ~20% going from $i_{775}$ to $H_{160}$ band, consistent with recent studies.
We then generate spatially resolved stellar mass maps using an empirical
relationship between $M_{*}/L_{H_{160}}$ and $(z_{850}-H_{160})$ and use these
to derive mass-weighted S\'ersic fits: the mass-weighted sizes are ~41% smaller
than their rest-frame $r$-band counterparts compared with an average of ~12% at
z~0. We attribute this evolution to the evolution in the $M_{*}/L_{H_{160}}$
and colour gradient. Indeed, as expected, the ratio of mass-weighted to
light-weighted size is correlated with the $M_{*}/L$ gradient, but is also
mildly correlated with the mass surface density and mass-weighted size. The
colour gradients $(\nabla_{z-H})$ are mostly negative, with a median value of
$\sim0.45$ mag dex$^{-1}$, twice the local value. The evolution is caused by an
evolution in age gradients along the semi-major axis ($a$), with $\nabla_{age}
= d \log(age) / d \log(a)$ $\sim-0.33$, while the survival of weaker colour
gradients in old, local galaxies implies that metallicity gradients are also
required, with $\nabla_{Z} = d \log(Z) / d \log(a)$ $\sim-0.2$. This is
consistent with recent observational evidence for the inside-out growth of
passive galaxies at high redshift, and favours a gradual mass growth mechanism,
such as minor mergers.
