Scaling Mass Profiles around Elliptical Galaxies Observed with Chandra and XMM-Newton

We investigated the dynamical structure of 53 elliptical galaxies using the Chandra archival X-ray data. In X-ray-luminous galaxies, temperature increases with radius and gas density is systematically higher at the optical outskirts, indicating the presence of a significant amount of the group-scale hot gas. In contrast, X-ray-dim galaxies show a flat or declining temperature profile against radius and the gas density is relatively lower at the optical outskirts. Thus, it is found that X-ray-bright and faint elliptical galaxies are clearly distinguished by the temperature and gas density profile. The mass profile is well scaled by a virial radius r200 rather than an optical half-radius re, is quite similar at (0.001-0.03)r200 between X-ray-luminous and dim galaxies, and smoothly connects to those profiles of clusters of galaxies. At the inner region of (0.001-0.01)r200 or (0.1-1)re, the mass profile well traces a stellar mass with a constant mass-to-light ratio of M/LB = 3-10 M☉/L☉. The M/LB ratio of X-ray-bright galaxies rises up steeply beyond 0.01r200 and thus requires a presence of massive dark matter halo. From the deprojection analysis combined with the XMM-Newton data, we found that X-ray-dim galaxies NGC 3923, NGC 720, and IC 1459 also have a high M/LB ratio of 20-30 at 20 kpc, comparable to that of X-ray-luminous galaxies. Therefore, dark matter is indicated to be common in elliptical galaxies; their dark matter distribution, as well as that of galaxy clusters, almost follows the NFW profile.