Measuring the diffuse optical light in Abell 1651

Using drift-scan data, a new approach to determining surface brightness pro files, 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 o f the BCG and the cluster are comparable. We find that the profile is consi stent with a de Vaucouleurs profile over the radial range probed. We also f ind 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 similar to 160 h for the cluster, which would be overestimated by similar to 20% without the BCG halo. Furthermore, the r elatively red color of the BCG at large radii suggests that recent disrupti on and tidal stripping of spirals and dwarf ellipticals do not contribute s ignificantly to the halo luminosity. The color and the form of the profile are consistent with a scenario in which the BCG forms from filamentary coll apse during the epoch of cluster formation, with relatively little evolutio n 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 cluste r core. Several knots of excess emission are found, but the total diffuse c omponent is constrained to contribute less than 5% of the cluster light.