A photometrically detected forming cluster of galaxies at redshift 1.6 in the goods field

We report the discovery of a localized overdensity at z similar to 1: 6 in the GOODS-South field, presumably a poor cluster in the process of formation. The three-dimensional galaxy density has been estimated on the basis of well-calibrated photometric redshifts from the multiband photometric GOODS-MUSIC catalog using the (2+1)-dimensional technique. The density peak is embedded in the larger scale overdensity of galaxies known to exist at z 1: 61 in the area. The properties of the member galaxies are compared to those of the surrounding field, and we find that the two populations are significantly different, supporting the reality of the structure. The reddest galaxies, once evolved according to their best-fit models, have colors consistent with the red sequence of lower redshift clusters. The estimated M-200 total mass of the cluster is in the range 1.3 x 10(14) -5.7 x 10(14) M-circle dot, depending on the assumed bias factor b. An upper limit for the 2-10 keV X-ray luminosity, based on the 1 Ms Chandra observations, is L-X = 0.5 x 10(43) erg s(-1), suggesting that the cluster has not yet reached the virial equilibrium.