MORE BOUND POPULATIONS AROUND CD GALAXIES

The dynamical and structural properties of galaxies in the cores of eight rich clusters have been measured to search for a population of objects that are bound to the central dominant galaxy. We report the positive detection of two such populations (A1991 and A2589) and the marginal detection of two more (A2107 and A2593). The combined sample is best fitted with a double Gaussian that contains 20% of core members with a characteristic σ of 120 km s-1 and 80% with a σ of 1200 km s-1. In addition, we report on the curious case of A2634 in which the cD has a peculiar velocity of 375 km s-1 but which appears to have a bound population when the mean cluster redshift is substituted for the cD redshift. Consequently, it appears that some clusters can contain a dynamically bound population without the presence of a centrally located cD. Further support for bound populations is that 80% of the low-velocity galaxies in the cluster core have luminosity profiles which are tidally truncated. In fact, the mean tidal radius we derive is 2.5 kpc which is less than the typical effective radius of these galaxies. This implies at least one close passage (R ≈ 15 kpc) near the cluster center in a crossing time. Comparison to template profiles estimates that 50% ± 12% of the luminosity of these core galaxies has been removed. The characteristic dynamical friction time scale in individual clusters ranges from 3.5 to 7 Gyr which results in 5-10L* of material available to the cD envelope over a Hubble time. In order to rectify the apparent contradiction between close passage, and yet apparent survival of the bound population galaxies, we propose that galaxies which are members of the bound population are on highly radial orbits with a typical apogee of 150 kpc.