Galaxy evolution in overdense environments at high redshift: passive
  early-type galaxies in a cluster at redshift 2
  We present a study of galaxy populations in the central region of the
IRAC-selected, X-ray detected galaxy cluster Cl J1449+0856 at z=2. Based on a
sample of spectroscopic and photometric cluster members, we investigate stellar
populations and morphological structure of cluster galaxies over an area of
~0.7Mpc^2 around the cluster core. The cluster stands out as a clear
overdensity both in redshift space, and in the spatial distribution of galaxies
close to the center of the extended X-ray emission. The cluster core region
(r<200 kpc) shows a clearly enhanced passive fraction with respect to field
levels. However, together with a population of massive passive galaxies mostly
with early-type morphologies, it also hosts massive actively star-forming,
often highly dust-reddened sources. Close to the cluster center, a
multi-component system of passive and star-forming galaxies could be the future
BCG still assembling. We observe a clear correlation between passive stellar
populations and an early-type morphology, in agreement with field studies at
similar redshift. Passive early-type galaxies in this clusters are typically a
factor 2-3 smaller than similarly massive early-types at z~0, but also on
average larger by a factor ~2 than their field analogs at z~2, lending support
to recent claims of an accelerated structural evolution in high-redshift dense
environments. These results point towards the early formation of a population
of massive galaxies, already evolved both in their structure and stellar
populations, coexisting with still-actively forming massive galaxies in the
central regions of young clusters 10 billion years ago.
