Cluster and cluster galaxy evolution history from IR to X-ray
  observations of the young cluster RX J1257.2+4738 at z=0.866
  The cosmic time around the z~1 redshift range appears crucial in the cluster
and galaxy evolution, since it is probably the epoch of the first mature galaxy
clusters. Our knowledge of the properties of the galaxy populations in these
clusters is limited because only a handful of z~1 clusters are presently known.
In this framework, we report the discovery of a z~0.87 cluster and study its
properties at various wavelengths. We gathered X-ray and optical data (imaging
and spectroscopy), and near and far infrared data (imaging) in order to confirm
the cluster nature of our candidate, to determine its dynamical state, and to
give insight on its galaxy population evolution. Our candidate structure
appears to be a massive z~0.87 dynamically young cluster with an atypically
high X-ray temperature as compared to its X-ray luminosity. It exhibits a
significant percentage ~90% of galaxies that are also detected in the 24micron
band. The cluster RXJ1257.2+4738 appears to be still in the process of
collapsing. Its relatively high temperature is probably the consequence of
significant energy input into the intracluster medium besides the regular
gravitational infall contribution. A significant part of its galaxies are red
objects that are probably dusty with on-going star formation.
