| Groups and clusters are slowly evolving habitats in which galaxies, primordial gas, and dark matter coexist. Groups are small, abundant, and enigmatic; clusters are rich, rare, and better understood. To elucidate the differences in the physics of groups and clusters, we construct a statistically complete sample of 260 nearby groups (0.01 < z < 0.03), 61 of which have detectable X-ray emission characteristic of a hot, gaseous atmosphere. We also measure 2424 new velocities for galaxies in the regions surrounding 41 of these groups.; The X-ray luminosity of the gas (LX) and the velocity dispersion of the galaxies (σ) in a group or cluster trace its gravitational binding energy in fundamentally independent ways. We show that although rich clusters and isolated galaxies form a tight, continuous LX − σ, relation spanning 8 orders of magnitude in luminosity, small groups of galaxies exhibit an enormous scatter around this relation. Our results suggest that interaction of the member galaxies with the X-ray atmosphere is important in some groups.; We use the velocity data to investigate the distribution of dark matter in the central regions of groups and clusters. Through a direct dynamical modeling of the joint distribution of galaxy positions and velocities, and without assuming that mass follows light, we show that the mass density in groups rises sharply towards the center (ρ ∝ r 1.5–1.9), whereas in rich clusters of galaxies it remains roughly constant (ρ ∝ r0–0.9). Finally, we show how the X-ray and optical data might be combined to constrain the orbital distribution of cluster dark matter halos. |
