Internal kinematics of galaxies in clusters. I - Velocity dispersions for elliptical galaxies in Coma and Virgo

This paper presents velocity dispersions and measurements of the Mg2 index for 30 ellipticals in the Coma Cluster and 23 ellipticals in the Virgo Cluster. The techniques of data acquisition and analysis are briefly described and the high internal accuracy of the measurements is demonstrated. The relationships among galaxy luminosity, velocity dispersion, and Mg2 index are presented and discussed with respect to the "second-parameter" question. The L-σ and L-Mg2 relations are used to estimate the difference in distance moduli of the Virgo and Coma clusters, and thus derive a value of the peculiar Virgocentric velocity of the Local Group of VV = 229±80 km s-1. This new value is opposed to the microwave dipole measurement of VV = 410±25 km s-1. A possible explanation for this discrepancy is considered.