4.3. Other Dark Matter Indicators for Galaxies

4.3.1. Hot Halos Around Elliptical Galaxies

X-ray observations of elliptical galaxies show that many have extended halos of X-ray emission whose overall size is a few times the optical radius of the galaxy. The origin of the hot gas that fills the halo is likely to come from within the optical radius. This hot gas has been driven out of the central potential, in which most of the stars are located, as a result of supernova heating of the cold gas from which those stars formed (see Matsumoto et al. 1997) Estimates, of the ratio of hot gas mass to surviving stellar mass in these ellipticals range from 0.1 - 1.0 (Forman et al. 1985). As ellipticals are observed today to have very low fractional mass contents of cold gas, it seems likely that any remaining cold gas after star formation ceased was heated and dynamically relocated to this hot halo.

These X-ray halos have observed radial gradients in x-ray flux that that indicate the halo gas mass is in hydrostatic equilibrium with the galactic potential. Although this X-ray gas is being collisionally cooled and falling back into the galaxy, the observed inflow velocity is much less than the ambient sound speed so that the condition of hydrostatic equilibrium is still applicable. Unlike the case of the isothermal sphere, however, there will be a temperature gradient and hence the mass distribution, M(r) can only be recovered if the density and temperature distributions are known. In general, the Einstein X-ray observations are of insufficient quality to define the temperature profile and thus M(r) remains unknown as does the actual mass of hot gas. In the case of the extended x-ray halo around M87, the data are of high quality and indicate a mass of 10¹³ M out to a radius of 150-200 kpc (Fiegelson et al. 1987 - see also Mould et al. 1987). These exceeds the stellar mass of M87 by at least an order of magnitude. In the general case of these halos, the condition of hydrostatic equilibrium combined with their very large extent indirectly suggests that the hot gas is prevented from escaping (and is therefore bound to the galaxy) by a dark matter halo.