An X - ray Cluster at Redshift 2 . 156 ?

ERRATUM A third possible explanation for the X-ray emission from the z = 2.156 radio galaxy 1138−262 is thermal emission from a very dense, sub-cluster 'halo', perhaps associated with the (forming) cD galaxy on a scale ≤ 100 kpc. For instance, increasing the gas density from 0.01 cm −3 to 0.1 cm −3 would decrease the required hot gas mass by the same factor, and could possibly alleviate constraints on cosmological structure formation models. The pressure in this gas would be very high (10 −9 dynes cm −2), comparable to the pressure in the optical line emitting nebulosity and to the minimum pressure in the radio source, and the cooling time would be short (≤ few×10 8 years). Circumstantial evidence for such very dense, hot gas enveloping some high z radio sources has been reviewed by Fabian et al. (1986). Fabian (1991) suggests that in some cases the 'inferred pressures are close to the maximum that can be obtained by gas cooling in a potential well of a galaxy' (ie. the cooling time ≈ gravitational free-fall time), and he designates such systems as 'maximal cooling flows', with implied cooling flow rates up to 2000 M ⊙ year −1. High resolution X-ray imaging with AXAF should be able to test whether 1138−262 has a normal cluster atmosphere, a 'maximal cooling flow', or an unusually X-ray loud AGN.