Baryonically Closed Galaxy Groups

Elliptical galaxies and their groups having the largest LX/LB lie close to the locus LX = 4.3 × 1043(LB/1011 LB, ☉)1.75 expected for closed systems having baryon fractions equal to the cosmic mean value, fb ≈ 0.16. The estimated baryon fractions for several of these galaxies/groups are also close to fb = 0.16 when the gas density is extrapolated to the virial radius. Evidently they are the least massive baryonically closed systems. Gas retention in these groups implies that nongravitational heating cannot exceed about 1 keV per particle, consistent with the heating required to produce the deviation of groups from the LX-T correlation for more massive clusters. Isolated galaxies/groups with X-ray luminosities significantly lower than baryonically closed groups may have undermassive dark halos, overactive central AGNs, or higher star formation efficiencies. The virial mass and hot gas temperatures of nearly or completely closed groups correlate with the group X-ray luminosities and the optical luminosities of the group-centered elliptical galaxy, i.e., Mvir ∝ L, an expected consequence of their merging history. The ratio of halo mass to the mass of the central galaxy for X-ray-luminous galaxies/groups is Mvir/M* ~ 80.