We detect four isolated , X-ray over-luminous ( L _ { x } > 2 \times 10 ^ { 43 } [ h / 0.5 ] ^ { -2 } erg s ^ { -1 } ) elliptical galaxies ( OLEGs ) in our 160 square degree 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 correspond to poor clusters , and the optical luminosity of the central galaxies ( M _ { R } < -22.5 + 5 \lg h ) is comparable to that of cluster cDs .
However , there are no detectable fainter galaxy concentrations around the central elliptical .
The mass-to-light ratio within the radius of detectable X-ray emission is in the range 250 - 450 M _ { \odot } / L _ { \odot } , 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 .
However , their high M / L values are difficult to explain in this scenario .
OLEGs must have been undisturbed for a very long time , which makes them the ultimate examples of systmes in hydrostatic equilibrium .
The number density of OLEGs is n = 2.4 ^ { +3.1 } _ { -1.2 } \times 10 ^ { -7 } ( h / 0.5 ) ^ { -3 } Mpc ^ { -3 } at the 90 % confidence .
They comprise 20 % of all clusters and groups of comparable X-ray luminosity , and nearly all galaxies brighter than M _ { R } = -22.5 .
The estimated contirubution of OLEGs to the total mass density in the Universe is close to that of T > 7 keV clusters .