X-Ray Overluminous Elliptical Galaxies: A New Class of Mass Concentrations in the Universe?

We detect four isolated, X-ray overluminous [Lx > 2 × 1043 -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 (MR < -22.5 + 5 log h) 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 × 10-7 (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 MR = -22.5. The estimated contribution of OLEGs to the total mass density in the universe is close to that of T > 7 keV clusters.

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