One of the Highest Redshift X-Ray Selected Clusters of Galaxies

An unusual double-lobed extended X-ray source with an angular size of 1.7 x 0.7 arcmin² and an X-ray flux of 2⋅10⁻¹⁴ erg cm⁻² s⁻¹ has been detected in the ROSAT ultra-deep HRI survey of the Lockman Hole. An R-band image from the Keck telescope shows only a marginal excess of galaxies brighter than 24.5 mag and Keck LRIS spectroscopy of 24 galaxies around the X-ray centroid did not yield a significant number of concordant redshifts. The brightest galaxy near the centre of the eastern emission peak is a gravitationally lensed arc at z=2.570, indicating that the X-ray object is a high-redshift lensing cluster of galaxies. This is the first arc, which is optically brighter than any of the galaxies in the lensing cluster. Recently, deep K′-band images of the region revealed a number of bright galaxies (K′ = 16-17) with very red colours (R−K′) > 6.3 - 6.7. From Bruzual & Charlot evolution models for elliptical galaxies we have estimated a redshift of z∼1.3. This is therefore one of the most distant X-ray selected clusters of galaxies known. Its X-ray luminosity, temperature and lensing mass are in the same range as for clusters found in the local universe.