Measuring the Diffuse Optical Light in Abell 1651

Using drift scan data, a new approach to determining surface brightnessprofiles, and techniques for detecting low surface brightness signals, we fitthe light profile of the brightest cluster galaxy (BCG) in the rich clusterAbell 1651 out to 670 h^{-1} kpc. This radius is a significant fraction of thevirial radius of the cluster (2 h^{-1} Mpc; Girardi et al. 1998), indicatingthat the sizes of the BCG and the cluster are comparable. We find that theprofile is consistent with a de Vaucouleurs profile over the radial rangeprobed. We also find that the integrated light profile of the BCG in Abell 1651contributes 36% of the total cluster light within 500 h^{-1} kpc. Including allluminous components, we obtain M/L_I~160 h for the cluster, which would beoverestimated by ~20% without the BCG halo. Furthermore, the relatively redcolor of the BCG at large radii suggests that recent disruption and tidalstripping of spirals and dwarf ellipticals do not contribute significantly tothe halo luminosity. The color and the form of the profile are consistent witha scenario in which the BCG forms from filamentary collapse during the epoch ofcluster formation, with relatively little evolution in the past 5 Gyrs(Dubinski 1998). We remove the BCG and other detected galaxies from the imageand construct a two dimensional surface brightness map of the cluster core.Several knots of excess emission are found, but the total diffuse component isconstrained to contribute less than 5% of the cluster light.