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A&A 439, 29-33 (2005)
DOI: 10.1051/0004-6361:20042112

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

Yasuhiro Hashimoto¹, J. Patrick Henry^{1, 2}, G. Hasinger¹, G. Szokoly¹ and M. Schmidt<sup>3

1</sup>  Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 85748 Garching, Germany
    e-mail: hashimot@mpe.mpg.de
²  Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822, USA
³  Palomar Observatory, California Institute of Technology, MS 320-47 Pasadena, CA 91125, USA

(Received 4 October 2004 / Accepted 23 February 2005 )

Abstract
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 20 15 region surrounding RX J1053.7+5735, we secured redshifts for six galaxies in the range 1.129 < z < 1.139, with a mean redshift . 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 . 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 , 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 ~ 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 .

Key words: galaxies: clusters: general -- galaxies: high-redshift -- infrared: galaxies -- X-rays: galaxies: clusters -- galaxies: evolution -- galaxies: stellar content

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