X-ray Emission from Elliptical Galaxies

Elliptical galaxies are generally luminous sources of X-ray radiation, and contain large amounts of hot, interstellar gas. In the brighter X-ray galaxies, the inferred masses of hot gas are consistent with those expected given the present rates of stellar mass loss. The required rates of heating of the gas are also roughly consistent with those expected from the motions of gas losing stars. X-ray observations, particularly X-ray spectra, require a low rate of Type Ia supernova heating and chemical enrichment in the gas. In the brightest X-ray galaxies, the cooling times in the gas are short, which suggests that the gas forms steady-state cooling flows. Steady cooling models explain most of the properties of the brighter X-ray galaxies, including their luminosities, the X-ray--optical correlation, their temperatures, and their surface brightness profiles. Although the optical and X-ray luminosities of early-type galaxies are strongly correlated, there is a large dispersion in this correlation. The origin of the emission in the X-ray faint ellipticals is less certain. All ellipticals appear to have a hard X-ray spectral component due to accreting binary systems. X-ray faint ellipticals also have a very soft X-ray component, which may be residual hot interstellar gas. The X-ray spectra of ellipticals indicate that the abundance of iron is well below the solar value, which implies that the rate of Type Ia supernova contamination is small. The abundances and evidence for gradient gradients suggest that stellar abundance gradients and inflow of the gas affect the X-ray spectra.