Astron. Astrophys. 317, 670-675 (1997)

5. Discussion

We analysed the kinematics of the core regions of 12 nearby galaxy clusters. The homogeneous redshift sample was nearly complete down to faint limits ( ) and was supplemented by magnitudes and rough morphological types for most of the galaxies. An analysis of substructure using velocity data revealed that 50% of the cluster cores harbour significant substructure, thus confirming that many clusters are not even relaxed in their inner regions, where effects like mass segregation and infalling groups of galaxies might be disturbing the virialization process.
After having merged the data to a sample of normalized galaxy velocities, magnitudes and types, we looked for type and type/luminosity segregation in velocity space. Previous findings (Binggeli et al. 1987; Sodré et al. 1989) about early type galaxies having lower velocity dispersions than late types are confirmed by the present analysis. Zabludoff & Franx (1993) found no such relation, on the opposite they claimed deviations in the velocity means between different types, concluding that there must be groups of spirals falling onto the cluster main body and distorting the distribution of velocities. Their findings should be considered complementary to ours, because of the different scale observed (R 0.5 h^-1 Mpc versus R 1.5 h^-1 Mpc).
Luminosity segregation in velocity space is also present, qualitatively and quantitatively in agreement with the findings of Biviano et al. (1992), who were using a larger, but more heterogeneous data sample. Moreover, there is a link between type and luminosity segregation. Only the brightest E and, possibly, S0-galaxies ( ) show clear signs of the phenomenon of luminosity segregation in velocity, which is probably related to two-body relaxation effects. These galaxies are responsible for the differences in velocity dispersion between early and late galaxy types, plausibly representing the fraction of galaxy population in clusters which have undergone significant late dynamical evolution. On the other side, S-galaxies show no sign of luminosity segregation, as is expected from objects that are still infalling onto the cluster main body and are presumably crossing the core region for the first time.
It remains uncertain whether the effect is due to dynamical friction or comes from the fact that galaxies which are the result of repeated mergers will tend to have velocities closer to the cluster mean. In both cases, strong evolution in the dynamics of the oldest cluster population can be seen. Capelato et al. (1981) found several hints for the presence of partial equipartition of galaxy kinetic energies in clusters. If the /L ratio is close to constant for E-galaxies, then our findings support their view that only the most massive galaxies had had enough time to slow down.

© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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