Dynamical Properties of Compact Groups of Galaxies

We present radial velocities for 457 galaxies in the 100 Hickson compact groups. More than 84% of the galaxies measured have velocities within 1000 km s^-1^ of the median velocity in the group. Here 92 groups have at least three accordant members, and 69 groups have at least four. The radial velocities of these groups range from 1380 to 41,731 km s^- 1^ with a median of 8889 km s^-1^, corresponding to a median distance of 89 h^-1^ Mpc. The redshift distribution of the groups is consistent with their having a uniform space distribution, and a luminosity function characterized by a Schechter function with M^*^ = -20.6 and α = -0.2. The apparent space density of these systems ranges from 300 to as much as 10^8^ h^2^ Mpc^-2^, which exceeds the densities in the centers of rich clusters. The median projected separation between galaxies is only 39 h^-1^ kpc, comparable to the sizes of the galaxies themselves. The median radial velocity dispersion is 200 km s^-1^, comparable to values reported for loose groups. This corresponds to an intrinsic three- dimensional velocity dispersion of 331 km s^-1^. The median galaxy crossing time ranges from 0.001H^-1^_0_ to 8.7H^-1^_0_ with a median of 0.016H^-1^_0_. A significant correlation is found between crossing time and the fraction of gas-rich galaxies in the groups, and a weak anticorrelation is found between crossing time and the luminosity contrast of the first- ranked galaxy. Derived mass-to-light ratios M/L for the groups range up to 10^3^, with a median of 50 h. This is about 7 times the median M/L reported for individual galaxies in the groups and strongly suggests that as much as ~85% of the mass of a group, in the region interior to the galaxy orbits, consists of dark matter.