A COMPARISON OF DIRECT AND INDIRECT MASS ESTIMATES FOR DISTANT CLUSTERS OF GALAXIES

We present weak lensing results for 12 distant clusters determined fro m images obtained with the refurbished Hubble Space Telescope. We dete ct the signature of gravitational lensing in 11 of the 12 clusters; th e clusters span nearly 1 order of magnitude in lensing strength. The s ample thus provides an excellent database for correlating direct mass estimates based on lensing with indirect ones that rely on baryonic tr acers. We examine the correlation between the cluster X-ray luminositi es and the mean gravitational shear strengths and develop a model that allows us to predict the relationship expected from the properties of clusters. After allowing for various observational effects, we fmd th at the predicted correlation is a reasonable match to the available da ta, indicating that there has been little evolution in the X-ray lumin osity-central mass relationship between z similar to 0.4 and now. We d iscuss the implications of this result in the context of the evolution of the X-ray luminosity function found by earlier investigators. The comparison between shear amplitudes and velocity dispersions, estimate d from a modest sample of members (similar to 30), reveals a discrepan cy in the sense that these velocity dispersions are typically overesti mated by factors of about 50%. This supports earlier suggestions that high dispersions measured for distant clusters may be seriously affect ed by both unidentified substructure and outliers. Combining our shear -based mass estimates with morphologically based luminosity estimates, we determine mass-to-light ratios of M/L(V)(all)=180(-110)(+210)h (M/ L) for the entire population and 620(-240)(+250) h for spheroidal popu lations, in which the evolutionary effects can be best treated. We arg ue that this provides an upper bound to the present-day cluster mass-t o-light ratio corresponding to Omega similar to 0.4. Our results demon strate the important role weak gravitational lensing can play in the s tudy of the evolution of distant clusters, as the most direct and leas t biased probe of their growth.