1995PASP..107.1259P

Compact groups of galaxies typically contain fewer than ten galaxies, separated from one another on the plane of the sky by a galaxy diameter or less. A continuing question about these groups is why we see so many of them even though galaxy interactions should quickly destroy the individual galaxies and result in a single merger remnant. The presence of tidally distorted galaxies in these groups implies that they are not simply projections of unrelated galaxies on the sky. Hickson compact groups (HCGs) form a well-defined sample that can be examined for clues to this puzzle.

We used deep X-ray (ROSAT PSPC) and optical observations of twelve Hickson groups to explore their interaction histories. Diffuse X-ray emission (of luminosity 10^41-42 erg s^-1 is seen primarily in elliptical-rich HCGs, implying that these groups contain gas at a temperature of kT ∼0.9 keV. Total and gas masses obtained from the X-ray data show that these groups have baryon fractions similar to those of clusters of galaxies, but gas-to-stellar-mass ratios two orders of magnitude smaller. Deep, multicolor photometry of the same sample shows no correlation between the presence of tidal features in group galaxies and X-ray emission in a group; however, two bright ellipticals in the sample not previously thought to be merger remnants have peculiar color structures and extremely low X-ray luminosities which may result from mergers.

HCG 94 is an exceptional group: extremely X-ray-luminous, it is the only group in our sample with diffuse optical light in the group potential (the other groups have an upper limit of 1/3 L*, while the light in HCG 94 has a luminosity of 7 L*). High-resolution X-ray observations show that the diffuse hot gas and the diffuse light trace the same potential in this group, implying that this potential is changing slowly. Galaxy-formation simulations in a cold-dark-matter-dominated universe create groups that have properties similar to X-ray-bright HCGs; however, the simulated groups are not physically compact for most of the time that they are X-ray-luminous.