The structure and dynamics of Abell 2670 – I. M/L ratio and orbital anisotropy

We present a faint photometric and spectroscopic study of 303 galaxies centred on the rich cD cluster Abell 2670 (z = 0.076). Redshifts have been obtained for complete galaxy samples extending up to 4.8 Mpc from the cluster centre and a velocity dispersion profile for the cluster members has been determined with good precision out to ∼3 Mpc. We discuss the possibility of subclustering in the galaxy distribution, but conclude that, at present, there is no firm statistical evidence supporting this, other than a curious asymmetry of the galaxy distribution around the cD. For dynamical models with constant M/L ratio, substantial radial anisotropy is required in the galaxy orbits in order to reproduce the observed decline in velocity dispersion with radius. A central minimum in the dispersion profile can also be modelled if there is a circularization of orbits in the cluster core. More general models where the M/L is allowed to vary with radius have been additionally constrained using the emissivity profile of the hot X-ray gas in the cluster to derive plausible distributions for the dark matter. We find that the temperature gradient in the X-ray plasma must be less than the adiabatic limit but that the gas cannot be isothermal. If the dark matter is more clustered than the galaxies, the orbits must be predominantly tangential. More likely, the dark matter is less clustered but our observations rule out dark matter scale lengths of more than ∼2.5 times that of the visible material.