A Spitzer-Selected Galaxy Cluster at z=1.62
  We report the discovery of a galaxy cluster at z=1.62 located in the Spitzer
Wide-Area Infrared Extragalactic survey XMM-LSS field. This structure was
selected solely as an overdensity of galaxies with red Spitzer/IRAC colors,
satisfying [3.6]-[4.5] > -0.1 AB mag. Photometric redshifts derived from Subaru
XMM Deep Survey (BViz-bands), UKIRT Infrared Deep Survey-Ultra-Deep Survey
(UKIDSS-UDS, JK-bands), and from the Spitzer Public UDS survey (3.6-8.0 micron)
show that this cluster corresponds to a surface density of galaxies at z ~ 1.6
that is more than 20 sigma above the mean at this redshift. We obtained optical
spectroscopic observations of galaxies in the cluster region using IMACS on the
Magellan telescope. We measured redshifts for seven galaxies in the range
z=1.62-1.63 within 2.8 arcmin (<1.4 Mpc) of the astrometric center of the
cluster. A posteriori analysis of the XMM data in this field reveal a weak (4
sigma) detection in the [0.5-2 keV] band compatible with the expected thermal
emission from such a cluster. The color-magnitude diagram of the galaxies in
this cluster shows a prominent red-sequence, dominated by a population of red
galaxies with (z-J) > 1.7 mag. The photometric redshift probability
distributions for the red galaxies are strongly peaked at z=1.62, coincident
with the spectroscopically confirmed galaxies. The rest-frame (U-B) color and
scatter of galaxies on the red-sequence are consistent with a mean
luminosity-weighted age of 1.2 +/- 0.1 Gyr, yielding a formation redshift z_f =
2.35 +/- 0.10, and corresponding to the last significant star-formation period
in these galaxies.
