Medusa spectroscopy of A400, A576, A1767, and A2124

Galaxy velocity data taken with the Steward Observatory multiple aperture fiber optic spectrograph are presented for four Abell clusters. The root-mean-square external errors in these velocities are about 100 km/s; accuracy which compares favorably with that obtained from single-object observations. It is expected that the recent adoption of a CCD detector should decrease external errors to about 50 km/s. All four of the clusters observed are known X-ray sources and the present data agree well with empirically derived velocity dispersion-X-ray luminosity relations for clusters of galaxies. Abell 400 is interesting in this regard, since both its X-ray luminosity and its velocity dispersion are quite small. Such objects are particularly important in determining the slope of the velocity dispersion-X-ray luminosity relation. The large microwave decrement observed in A576 was initially interpreted as due to Compton scattering of the microwave background by the X-ray-emitting intracluster gas. White and Silk have presented Einstein X-ray data which indicate that A576 contains too little gas to produce the observed microwave decrement by Compton scattering. The velocity dispersion obtained here for 47 members of this cluster strengthens their conclusion.