Dark matter and dynamics in the Hercules Cluster (A2151)

Abstract

We present new 21 cm observations of faint (15.7 < m_pg_ < 16.5) spiral galaxies in the Hercules cluster (A2151). These results, when combined with the large body of velocities available in the literature for Hercules, permit us to study the dynamics throughout the cluster core, out to a projected radius of 1.8 h^-1^ Mpc. We calculate the global dynamical mass of Hercules using two different, but related, methods: the virial theorem and the projected mass estimator. These masses lie in the range 3-9 x 10^14^ M_sun_. We investigate the importance of subclustering in A2151 by eye, by using the statistical test of Dressler & Shectman, and by considering the effects of the detected substructure on the dynamical mass determination. The clumpy distribution of galaxies is interpreted as a sign that the galaxies in the cluster have not reached dynamical equilibrium in the gravitational potential. which means that dynamical mass estimates are subject to significant systematic errors. Finally. we use the 21 cm line widths to estimate that the minimum total mass which is contained within the H I radii of the cluster galaxies is about 3 x 10^14^ h^-1^ M_sun_. This number is compared with the dynamical mass estimate for the entire cluster; we can then separate the contribution of dark matter inside and outside the H I envelopes of galaxies. The fraction of dark matter which cannot be associated with individual galaxies within their H I radius is about 90%-95%.