Einstein observations demonstrated that E and S0 galaxies may have large hot gaseous halos, dominating their X-ray emission. Central cooling flows similar to some galaxy clusters have been suggested and the X-rays have been used to trace the gravitational potential of these galaxies (see  and references therein). However, not all E and S0 galaxies are able to retain a large amount of hot ISM. For some galaxies an extensive halo is clearly displaced from the stellar component and in some cases extending to very large radii. This has been shown by ROSAT for several galaxies and studied in detail for NGC 4406 with XMM-Newton . Other galaxies show much fainter X-ray emission compared to the optical, which typically is coincident with the stellar body seen in the optical. From ROSAT hardness ratio arguments this emission has been attributed to LMXBs . This proposal was nicely confirmed by high resolution Chandra and HST observations that show X-ray point sources correlating with globular clusters in the galaxies (see, e.g., ).
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