  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 (< fewx10^{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_solar 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.
