The ROSAT deep survey -: VII.: RX J105343+5735:: A massive cluster at z=1.263

The eastern lobe of the unusual double-lobed extended X-ray source RX J105343+5735 is confirmed to be a massive cluster at high redshift. Deep optical and near-infrared imaging show an overdensity of galaxies in both X-ray lobes, including a significant excess of red galaxies (R - K >5.3) with colors typical of elliptical galaxies at z >1. We have used new photometry to place better constraints on the redshifts of the cluster galaxies. A Keck NIRSPEC spectrum of one of the bright central galaxies in the eastern lobe shows a narrow H alpha emission line at 1.485 microns, yielding a redshift of 1.263. The [O II] lambda 3727 Angstrom line from the gravitationally lensed arc is also detected, giving a redshift of 2.577 for the lensed galaxy and confirming prior measurements. The improbability of chance alignment and similarity of colors for the galaxies in the two X-ray lobes are consistent with the western lobe also being at z = 1.26. The system may thus represent a pair of clusters in the process of merging.