The complex velocity distribution of galaxies in Abell 1689:: implications for mass modelling

The Abell 1689 galaxy cluster has recently become a subject of intensive study. Thanks to its intermediate redshift (z = 0.183), its mass distribution can be reconstructed using numerous methods including gravitational lensing, galaxy kinematics and X-ray imaging. The methods used to yield conflicting mass estimates in the past, and recently the cluster mass distribution has been claimed to be in conflict with standard cold dark matter scenarios due to the rather large concentration and steep mass profile obtained from the detailed studies of Broadhurst et al. using lensing. By studying in detail the kinematics of about 200 galaxies with measured redshifts in the vicinity of the cluster, we show that the cluster is probably surrounded by a few structures, quite distant from each other, but aligned along the line of sight. We support our arguments by referring to cosmological N-body simulations and showing explicitly that distant, non-interacting haloes can produce entangled multi-peak line-of-sight velocity distributions similar to that in Abell 1689. We conclude that it is difficult to estimate the cluster mass reliably from galaxy kinematics, but the value that we obtain after applying a simple cut-off in velocity agrees roughly with the mass estimated from lensing. The complicated mass distribution around the cluster may, however, increase the uncertainty in the determination of the density profile shape obtained with weak lensing.