DARK-MATTER IN X-RAY-EMITTING ELLIPTIC GALAXIES

To investigate the dynamical mass distribution in elliptical galaxies, we study the gravitational equilibrium of spherically symmetric three-component models constituted by stars, hot diffuse gas, and dark matter. Assuming a constant mass-to-light ratio for the stars, described with an isotropic distribution function in phase space (King), an emission in the X-ray band from the hot gas, and a dark component in the form of massive fermions, we derive constraints on the dark matter distribution by using optical and X-ray brightness profiles. We find that, while optical profiles are not useful to discriminate between models with and without dark matter, fits to X-ray observations of high-luminosity ellipticals require the presence of a third dark component, mainly distributed in the outer radial regions, beyond the half-luminosity radius. The total amount of dark matter is, however, poorly determined because X-ray brightness profiles can be fitted by models with values of dark mass M(d) in the range 0.4-9 times the stellar mass M* inside R25. In any case, the lower limit for M(d)/M* can be weakened for low-luminosity ellipticals, for which X-ray observations can be fitted by models with a negligible amount of dark matter. A better determination of the ratio M(d)/M* is obtained if stellar kinematical (velocity dispersion) data are also considered. Limits on the mass of fermions composing the dark matter are also discussed, and they are found to be in agreement with previous particle mass determinations based on the analysis of rotation curves in spiral galaxies. The possibility of fitting all the available observational data by using two-component (stars and hot gas) models with the stellar component described by an anisotropic distribution function in the phase space is also explored. For high-luminosity ellipticals also these models cannot fit X-ray observations because the corresponding gas temperatures are systematically higher than the observed values.