The red sequence at birth in the galaxy cluster ClJ1449+0856 at z=2
  We use HST/WFC3 imaging to study the red population in the IR-selected, X-ray
detected, low-mass cluster Cl J1449+0856 at z=2, one of the few bona-fide
established clusters discovered at this redshift, and likely a typical
progenitor of an average massive cluster today. This study explores the
presence and significance of an early red sequence in the core of this
structure, investigating the nature of red sequence galaxies, highlighting
environmental effects on cluster galaxy populations at high redshift, and at
the same time underlining similarities and differences with other distant dense
environments. Our results suggest that the red population in the core of Cl
J1449+0856 is made of a mixture of quiescent and dusty star-forming galaxies,
with a seedling of the future red sequence already growing in the very central
cluster region, and already characterising the inner cluster core with respect
to lower density environments. On the other hand, the color-magnitude diagram
of this cluster is definitely different from that of lower-redshift (z<1)
clusters, as well as of some rare particularly evolved massive clusters at
similar redshift, and it is suggestive of a transition phase between active
star formation and passive evolution occurring in the proto-cluster and
established lower-redshift cluster regimes.
