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

We investigated the dynamical structure of 53 elliptical galaxies, based on the {\it Chandra} archival X-ray data. In X-ray luminous galaxies, a temperature increases with radius and a gas density is systematically higher at the optical outskirts, indicating a 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 $r_{200}$ rather than an optical-half radius $r_e$, and is quite similar at $(0.001-0.03)r_{200}$ between X-ray luminous and dim galaxies, and smoothly connects to those of clusters of galaxies. At the inner region of $(0.001-0.01)r_{200}$ or $(0.1-1)r_e$, the mass profile well traces a stellar mass with a constant mass-to-light ratio of $M/L_{\rm B}=3-10(M_{\odot}/L_{\odot})$. $M/L_{\rm B}$ ratio of X-ray bright galaxies rises up steeply beyond $0.01r_{200}$, and thus requires a presence of massive dark matter halo. From the deprojection analysis combined with the {\it XMM-Newton} data, we found that X-ray dim galaxies, NGC 3923, NGC 720, and IC 1459, also have a high $M/L_{\rm B}$ 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, and their distribution almost follows the NFW profile, as well as galaxy clusters.