Hot gas in ellipticals: hydrostatics. X-ray data have established the presence of substantial amounts of gas at ~10 7 K in ellipticals, as well as spread throughout clusters. The temperature derives from the kinetic energy of stars, from which much of the gas must have come by enrichment arguments). Under these conditions, the gas cools very slowly (except under the special conditions of a cooling flow) so it stays put for long times. This allows a hydrostatic trace of the mass distribution, since the observed gas distribution must be stationary in the galaxy potential. In a spherical potential, the pressure P must balance overlying material: or the equivalent differential form. P(R) may be obtained from multiband X-ray data; as it happens, the temperature is less certain than the density. Ellipticals seem to have halos much like those suggested from flat rotation curves in spirals. Some dwarfs may also have dark matter, based on radial-velocity dispersions of their brightest carbon stars (Aaronson 1983 ApJL 266, L11). Here progress is hard work - one must go star by
This is the end of the preview.
access the rest of the document.