1999ApJ...520L...1V

We detect four isolated, X-ray overluminous [L_x>2x10⁴³ h/0.5^–2 ergs.s^–1] elliptical galaxies (OLEGs) in our 160 deg² ROSAT PSPC survey. The extent of their X-ray emission, total X-ray luminosity, total mass, and mass of the hot gas in these systems corresponds to poor clusters, and the optical luminosity of the central galaxies (M_R←22.5+5logh) is comparable to that of cluster cD galaxies. However, there are no detectable fainter galaxy concentrations around the central elliptical galaxy. The estimated mass-to-light ratio within the radius of detectable X-ray emission is in the range of 250-450 M_☉/L_☉, which is 2-3 times higher than typically found in clusters or groups. These objects can be the result of galaxy merging within a group. OLEGs must have been undisturbed for a very long time, which makes them the ultimate examples of systems in hydrostatic equilibrium. The number density of OLEGs is n=2.4^+3.1_–1.2x10^–7 (h/0.5)^–3 Mpc^–3 at the 90% confidence level. They comprise 20% of all clusters and groups of comparable X-ray luminosity, and nearly all field galaxies brighter than M_R=-22.5. The estimated contribution of OLEGs to the total mass density in the universe is close to that of T>7 keV clusters.