Gravitational lensing of distant field galaxies by rich clusters - II. Cluster mass distributions

We construct a photometric catalogue of very faint galaxies (<e1>I<~</e1>25.5) using deep CCD images taken with the 4.2-m William Herschel telescope of fields centred on two distant X-ray-luminous clusters: 1455+22 (z_cl=0.26) and 0016+16 (zcl=0.55). Using a non-parametric procedure developed by Kaiser & Squires, we analyse the statistical image distortions in our samples to derive two-dimensional projected mass distributions for the clusters. The mass maps of 1455+22 and 0016+16 are presented at effective resolutions of 135 and 200 kpc, respectively (for H0=50 km s^-1 Mpc^-1, q_0=0.5), with peak signal-to-noise ratios per resolution element of 17 and 14. Although the absolute normalization of these mass maps depends on the assumed redshift distribution of the <e1>I<~</e1>25.5 field galaxies used as probes, the maps should be reliable on a relative scale and will trace the cluster mass regardless of whether it is baryonic or non-baryonic. We compare our 2D mass distributions on scales up to ~1 Mpc with those defined by the spatial distribution of colour-selected cluster members and with deep high-resolution X-ray images of the hot intracluster gas. Despite the different cluster structures - one is cD-dominated and the other is not - in both cases the form of the mass distribution derived from the lensing signal is strikingly similar to that traced by both the cluster galaxies and the hot X-ray gas. We find some evidence for a greater central concentration of dark matter with respect to the galaxies. The overall similarity between the distribution of total mass and that defined by the baryonic components presents a significant new observational constraint on the nature of dark matter and the evolutionary history of rich clusters.