A low mass cluster of extremely red galaxies at z=1.10 in the GOODS Southern Field

We have studied the spatial clustering of high redshift (z > 1) extremely red objects (EROs) as a function of photometric redshift in the GOODS Southern Field using public data. A remarkable overdensity of extremely red galaxies (I-Ks > 4) is found at an average photometric redshift z=1.10. Nine objects (six are EROs) within 50 arcsec of the brightest infrared galaxy in this overdensity present spectroscopic redshifts in the range 1.094 < z < 1.101 with a line-of-sight velocity dispersion of 433 km/s typical of an Abell richness class R=0 cluster. Other potential members of this cluster, designated as GCL J0332.2-2752, have been identified using photometric redshifts and the galaxy density profile studied as a function of radius. The mass of the cluster is preliminary estimated at M ~ 5-7 x 10^{13} M_{\sun}. Using available Chandra data, we limit the rest-frame X-ray luminosity of the cluster to less than L_X = 7.3 x 10^{42} erg/s (0.5-2.0 keV). This low-mass, low L_X cluster at z > 1 shows the potential of EROs to trace clusters of galaxies at high redshift.