2005A&A...439...29H

We present a study of the galaxy population in the cluster RX J1053.7+5735, one of the most distant X-ray selected clusters of galaxies and one which also shows an unusual double-lobed X-ray morphology, indicative of a possible equal-mass cluster merger. The cluster was discovered during the ROSAT deep pointings in the direction of the Lockman Hole. Using Keck-DEIMOS spectroscopic observations of galaxies in the 2.0'x1.5' region surrounding RX J1053.7+5735, we secured redshifts for six galaxies in the range 1.129<z<1.139, with a mean redshift <z≥1.134. This mean redshift agrees well with the cluster X-ray redshift previously estimated from the cluster X-ray Fe-K line, confirming the presence of a cluster at z∼1.135. Galaxies with concordant redshifts are located in both eastern and western sub-clusters of the double cluster structure, which indicates that both sub-clusters are at similar redshifts. This result is also consistent with a previous claim that both eastern and western X-ray lobes have similar X-ray redshifts. Based on their separation of ∼250kpc, these results support the interpretation that RX J1053.7+5735 is an equal-mass cluster merger taken place at z∼1, although further direct evidence for a dynamical state of the cluster is needed to make a more definitive statement about the cluster merging state. The six galaxies have a line-of-sight velocity dispersion Δv∼650km/s, and all six show clear absorption features of CaII H and K and several Balmer lines that are typical of early galaxies at the present epoch, in agreement with their I-K colors. A color-magnitude diagram, constructed from deep optical/NIR observations of the RX J1053.7+5735 field, shows a clear red color sequence. There is an indication that the red sequence in RX J1053.7+5735 lies ∼0.3 to the blue of the Coma line, qualitatively consistent with previous studies investigating other clusters at z∼1.