Measuring the diffuse optical light in Abell 1651

Using drift-scan data, a new approach to determining surface brightness profiles, and techniques for detecting low surface brightness signals, we fit the light profile of the brightest cluster galaxy (BCG) in the rich cluster Abell 1651 out to 670 h^-1 kpc. This radius is a significant fraction of the virial radius of the cluster (2 h^-1 Mpc), indicating that the sizes of the BCG and the cluster are comparable. We find that the profile is consistent with a de Vaucouleurs profile over the radial range probed. We also find that the integrated light profile of the BCG in Abell 1651 contributes 36% of the total cluster light within 500 h^-1 kpc. Including all luminous components, we obtain M/L_I ∼ 160 h for the cluster, which would be overestimated by ∼ 20% without the BCG halo. Furthermore, the relatively red color of the BCG at large radii suggests that recent disruption and tidal stripping of spirals and dwarf ellipticals do not contribute significantly to the halo luminosity. The color and the form of the profile are consistent with a scenario in which the BCG forms from filamentary collapse during the epoch of cluster formation, with relatively little evolution in the past 5 Gyr. We remove the BCG and other detected galaxies from the image and construct a two-dimensional surface brightness map of the cluster core. Several knots of excess emission are found, but the total diffuse component is constrained to contribute less than 5% of the cluster light.