An X-Ray Cluster at Redshift 2.156?

We present X-ray observations of the narrow-line radio galaxy 1138-262 at z =2.156 with the High Resolution Imager (HRI) on ROSAT . Observations at other wave bands, and in particular extremely high values of Faraday rotation of the polarized radio emission, suggest that the 1138-262 radio source is in a dense environment, perhaps a hot, cluster-type atmosphere. We detect X-ray emission from the vicinity of 1138-262, and we discuss possible origins for this emission. The X-ray, optical, and radio data all favor thermal emission from a hot cluster atmosphere as the mechanism responsible for the X-rays, although we cannot rule out a contribution from the active nucleus. If this interpretation is correct, then 1138-262 becomes the most distant, by far, of known X-ray-emitting clusters. The X-ray luminosity for 1138-262 is 6.7±1.3 × 10 44 ergs s -1 for emitted energies between 2 and 10 keV.