5.4. Results from Modelling X-ray Data
The hot X-ray emitting gas in luminous ellipticals is another means to estimate the gravitational potential. This requires measuring the X-ray emissivity and temperature. Typical temperature profiles are nearly constant. Some problems are that point sources must be removed from the emission maps, requiring high spatial resolution, and that the assumed hydrostatic equilibrium may not be strictly fulfilled. However, any gas motions will have velocities of order the sound speed, so the error made with this assumption is not likely to be very large.
X-ray mass measurements have been made regularly as new X-ray missions were flown, but have tended to concentrate on a small number of galaxies. With the new high-spatial and spectral resolution and high-sensitivity data from Chandra and XMM this situation may soon change. In the detailed analyses sofar, M / L ratios of ~ 50 at 10's of kpc have been found in, e.g., NGC 1399, M87, NGC 4472, NGC 4636, NGC 507, NGC 720 [6, 39, 43]; these galaxies are often the centers of galaxy clusters, however. Ubiquitous halos have furthermore been found in studies of the family properties of ellipticals from X-ray data .