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 AC114 ($z=0.31$)obtained with Wide Field and Planetary Camera II onboard the Hubble SpaceTelescope. This image considerably extends our knowledge of the lensingproperties of the cluster beyond those derived by Smail \et (1995a) from asingle WF/PC-1 pointing. In conjunction with published ground-basedspectroscopy, we utilise several newly-discovered multiple images to constructan improved mass model for the central regions of the cluster. Using thismodel, we apply the methodology introduced by Natarajan & Kneib (1997) tointerpret local perturbations to the cluster shear field on small scalesresulting from mass associated with individual cluster galaxies. We use thelensing signal to place new constraints on the average mass-to-light ratio andspatial extents of the dark matter halos associated withmorphologically-classified early-type cluster members. We find that the totalmass of a fiducial $L^*$ cluster spheroidal galaxy is largely contained within$\sim$ 15 kpc radius halo ($\sim$~8--10 $R_e$) with a mass-to-light ratio${M/L_V} \sim {15^{+10}_{-4}} $ (90% c.l., $h=0.5$) in solar units within thisradius. Comparisons with similar estimates for fieldgalaxies indicates that thecluster galaxies in AC114 may possess less extensive and less massive halos. Wediscuss the consequences of our result in the context of models for thedynamical evolution of cluster galaxies and the observational prospects forextending this analysis.Comment: LaTeX file, 3 figures + 1 Table, 11 pages, ApJ in pres