Galaxy populations of double cluster RX J1053.7+5735 at z = 1.13

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 × 1. 5 region surrounding RX J1053.7+5735, we secured redshifts for six galaxies in the range 1.129 < z < 1.139, with a mean redshiftz� = 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 ∼250 kpc, 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 ∆v ∼ 650 km s −1 , 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.