We detect four isolated, X-ray overluminous [ L x > 2 × 10 43 -2 ergs s -1 ] elliptical galaxies (OLEGs) in our 160 deg 2 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 + 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 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.
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Vikhlinin et al. (1999) studied this question.
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