The Mass-to-Light Ratio of Early-Type Galaxies: Constraints from Gravitational Lensing in the Rich Cluster AC 114

We present a new wide-field image of the distant cluster AC 114 (z = 0.31) obtained with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. This image considerably extends our knowledge of the lensing properties of the cluster beyond that derived earlier from a single WFPC1 pointing. In conjunction with published ground-based spectroscopy, we utilize several newly discovered multiple images to construct an improved mass model for the central regions of the cluster. Using this model, we apply the methodology introduced by Natarajan & Kneib to interpret local perturbations to the cluster shear field on small scales resulting from mass associated with individual cluster galaxies. We use the lensing signal to place new constraints on the average mass-to-light ratio and spatial extents of the dark matter halos associated with morphologically classified early-type cluster members. We find that the total mass of a fiducial L^* cluster spheroidal galaxy is largely contained within ~15 kpc radius halo (~8-10R_e), with a mass-to-light ratio M/L_V~15⁺¹⁰_-4 (90% confidence level) in solar units within this radius. Comparisons with similar estimates for field galaxies suggest that the cluster galaxies in AC 114 may possess less extensive and less massive halos. Additionally, we find some indication that, at a fixed luminosity, S0 galaxies are less extended than ellipticals, suggesting a difference in the efficiency of tidal stripping of different galaxy types. We discuss the consequences of our results in the context of models for the dynamical evolution of cluster galaxies and the observational prospects for extending such analyses.

Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.