X-Ray Observations and the Structure of Elliptical Galaxies

We compare optical and high quality x-ray data for three bright ellipticalsin the Virgo cluster, NGC 4472, 4649, and 4636. The distribution of total massin NGC 4472 and 4649 determined from x-ray data is sensitive to the stellarmass over a considerable range in galactic radius extending to r = r_e, theeffective radius. The agreement of x-ray and optically-determined stellarmasses provides a unique verification of the stellar mass to light ratio whichis essentially constant over 0.1 < r/r_e < 1. However, dark matter in NGC 4636is important at all radii > 0.35r_e. Evidently the dark to stellar mass ratiovaries in quite different ways in ellipticals of comparable optical luminosity,implying the radial structure of dark halos may not be universal. There is someevidence 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 globaltemperature structure in the hot interstellar medium of many recently observedellipticals is very similar, reaching a maximum near 3 - 4 r_e. This feature,which may suggest a new structural scale, is inconsistent with currenttheoretical gas dynamical models.Comment: 7 pages in AASTEX LaTeX with 2 figures; accepted by Astrophysical Journal Letter