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 ( \hbox { $ [ 3.6 ] $ } - \hbox { $ [ 4.5 ] $ } ) _ { \mathrm { AB } } > -0.1 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 µm ) show that this cluster corresponds to a surface density of galaxies at z \approx 1.6 that is > 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 \pm 0.1 Gyr , yielding a formation redshift \overline { z _ { f } } = 2.35 \pm 0.10 , and corresponding to the last significant star-formation period in these galaxies .