Greatly Extended X-Ray Emission around the Elliptical Galaxy NGC 4636 Observed with [ITAL]ASCA[/ITAL]

ASCA observations of NGC 4636 and a southern region have revealed extended X-ray emission to a radius of about 300 kpc from the galaxy. The symmetric nature of the observed surface brightness around NGC 4636 indicates its association with this galaxy rather than with the Virgo Cluster. Model-independent estimation of the gravitational mass profile shows a flattening at a radius of ~20 kpc, where the total mass reaches ~4 × 10 11 M ☉ and a mass-to-light ratio of ~22. The mass still increases to larger radii, reaching 9 × 10 12 M ☉ with a mass-to-light ratio of 300 at ~300 kpc from NGC 4636. These features suggest the presence of a galaxy group surrounding NGC 4636. If such optically dark groups are common among X-ray-bright ellipticals, it would explain the very large scatter in their X-ray luminosities with similar optical luminosities.