The Mass Profile of the Coma Galaxy Cluster

We use a new redshift survey complete to m_R=15.4 within 4.25 deg from the center of the Coma cluster to measure the mass profile of the cluster to r=5.5 Mpc/h. We extend the profile to r=10 Mpc/h with a further sample complete to m_R=15.4 in 42% of the area within a 10 deg radius and to m_{Zw}=15.5 in the remaining area. Galaxies within this region are falling onto the cluster on moderately radial orbits and thus do not satisfy virial equilibrium. Nonetheless, identification of the caustics in redshift space provides an estimate of the gravitational potential at radius r and hence of the system mass, M(<10 Mpc/h)=(1.65+-0.41)10^{15} M_\odot/h (1-sigma error). Previous mass estimates derived from optical and X-ray observations are limited to r<2.5 Mpc/h. Our mass profile is consistent with these estimates but extends to distances four times as large. Over the entire range, the mass increases with r at the rate expected for a Navarro, Frenk & White (1997) density profile.