X-Ray Observations and the Structure of Elliptical Galaxies

Abstract

We compare optical and high quality x-ray data for three bright ellipticals in the Virgo cluster, NGC 4472, 4649, and 4636. The distribution of total mass in NGC 4472 and 4649 determined from x-ray data is sensitive to the stellar mass over a considerable range in galactic radius extending to r = r_e, the effective radius. The agreement of x-ray and optically-determined stellar masses provides a unique verification of the stellar mass to light ratio which is essentially constant over 0.1 < r/r_e < 1. However, dark matter in NGC 4636 is important at all radii > 0.35r_e. Evidently the dark to stellar mass ratio varies in quite different ways in ellipticals of comparable optical luminosity, implying the radial structure of dark halos may not be universal. There is some evidence in NGC 4636 for additional support of the hot interstellar gas at r < 0.35 r_e; either a field B = 10^{-4} G or a small (but mechanically unstable) central region of high gas temperature (T = 10^7 K) is required. The global temperature structure in the hot interstellar medium of many recently observed ellipticals is very similar, reaching a maximum near 3 - 4 r_e. This feature, which may suggest a new structural scale, is inconsistent with current theoretical gas dynamical models.