X-Ray observations of galaxies and clusters can, in principle, trace the binding mass in these systems. I review some of the relevant work. The mass of hot gas in rich clusters is comparable to or exceeds the mass in visible stars. This proportion of gas to stellar material could be universal, although there is no direct evidence that it must be. Studies of the distribution of the gas indicate the presence of dark matter in the envelopes of some dominant cluster galaxies, most notably M87. The M/LB values increase with radius to values of ∼ 400–600 M⊙/L⊙. Uncertainties in the temperature distribution of the gas have hampered these analyses and have made it difficult to draw definitive conclusions about the binding mass in clusters. Recent work on Coma suggests that M/L is falling with radius and the total M/L for the cluster may be as low as ∼ 120. Studies of early type galaxies show that many contain hot gas with temperatures ∼107 K. There is evidence for the existence of cooling flows, and gravity rather than supernovae may be the dominant source of energy that heats the gas. The deduced binding masses for several bright galaxies are uncertain because of the unknown temperature profiles. Values of M/LB ≃ 20–30 within ∼ 30–40 kpc are indicated if one assumes isothermality, but values as low as 5 and as high as 100 are allowed. With better models one may be able to reduce these uncertainties.
A note on temperature profiles of rich clusters of galaxies