X-Ray Observations and the Structure of Elliptical Galaxies

We compare optical and high-quality X-ray data for three bright elliptical galaxies 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 the range 0.1 ≲ r / r e ≲ 1. However, for NGC 4636 the dark matter is important at all radii ≳0.35 r e . Evidently, the dark to stellar mass ratio varies in quite different ways in elliptical galaxies of comparable optical luminosity, implying that 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 (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 elliptical galaxies is very similar, reaching a maximum near 3 r e -4 r e . This feature, which may suggest a new structural scale in these galaxies, is inconsistent with current theoretical gasdynamical models.