Probing the Distribution of Dark Matter in the A901/902 Supercluster with Weak Lensing

We present a weak-shear analysis of the A901/902 supercluster, composed of three rich clusters at z = 0.16. Using a deep R-band image from the 0.°5 × 0.°5 MPG/ESO Wide Field Imager together with supplementary B-band observations, we build up a comprehensive picture of the light and mass distributions in this region. We find that, on average, the light from the early-type galaxies traces the dark matter fairly well, although one cluster is a notable exception to this rule. The clusters themselves exhibit a range of mass-to-light (M/L) ratios, X-ray properties, and galaxy populations. We attempt to model the relation between the total mass and the light from the early-type galaxies with a simple scale-independent linear biasing model. We find M/LB = 130h for the early-type galaxies with zero stochasticity, which, if taken at face value, would imply Ωm < 0.1. However, this linear relation breaks down on small scales and on scales equivalent to the average cluster separation (~1 Mpc), demonstrating that a single M/L ratio is not adequate to fully describe the mass-to-light relation in the supercluster. Rather, the scatter in M/L ratios observed for the clusters supports a model incorporating nonlinear biasing or stochastic processes. Finally, there is a clear detection of filamentary structure connecting two of the clusters, seen in both the galaxy and dark matter distributions, and we discuss the effects of cluster-cluster and cluster-filament interactions as a means to reconcile the disparate descriptions of the supercluster.