THE DYNAMICS OF THE OUTER REGIONS OF THE COMA CLUSTER

We apply the spherical infall model to compare the observed distributi on of radial velocities of galaxies around Coma with predictions that are based on optical and X-ray overdensity profiles. Two optically sel ected galaxy catalogues, the CfA redshift survey and a UKST plate scan ned with the APM, are used to determine the over-density profile of th e cluster. The two sets of predicted caustics in the angular separatio n-redshift diagram are compared with a radial velocity catalogue which we have extended significantly in the range between 1-degrees and 4-d egrees from the centre of the Coma cluster. In total, we have measured 98 new redshifts. Despite almost doubling the number of radial veloci ties in this interval, we fail to find the caustics with any degree of certainty. Using recent X-ray-based density profiles of Coma, we deri ve an independent prediction of the caustics in redshift space. The ve locities predicted using these X-ray profiles are in general agreement with the observed velocity distribution. We also investigate the effe ct that non-spherical mass distributions have on the velocity field in the halo. For a velocity field that is constant on concentric ellipso ids, we show that the caustics are blurred, providing a natural explan ation of why we do not see the caustics in Coma.