Detection of Weak Lensing by a Cluster of Galaxies atz= 0.83

We report the detection of weak gravitational lensing of faint, distantbackground galaxies by the rich, X-ray luminous cluster of galaxies MS1054-03at z=0.83. This is the first measurement of weak lensing by a bona fide clusterat such a high redshift. We detect tangential shear at the 5% - 10% level overa range of radii 50'' < r < 250'' centered on the optical position of thecluster. Two-dimensional mass reconstruction using galaxies with 21.5 < I <25.5 shows a strong peak which coincides with the peak of the smoothed clusterlight distribution. Splitting this sample by magnitude (at I = 23.5) and color(at R-I = 0.7), we find that the brighter and redder subsamples are only veryweakly distorted, indicating that the faint blue galaxies (FBG's), whichdominate the shear signal, are relatively more distant. The derived clustermass is quite sensitive to the N(z) for the FBG's. At one extreme, if all theFBG's are at z_s = 3, then the mass within a $0.5h^{-1}$Mpc aperture is $(5.9\pm 1.24)\times 10^{14}$\h1 $M_\odot$, and the mass-to-light ratio is $M/L_V =350 \pm 70 h$ in solar units. For $z_s = 1.5$ the derived mass is $\sim$70\%higher and $M/L \simeq 580 h$. If $N(z)$ follows the no evolution model (inshape) then $M/L \simeq 800h$, and if all the FBG's lie at $z_s\la 1$ therequired $M/L$ exceeds $1600h$. These data provide clear evidence that large,dense mass concentrations existed at early epochs; that they can be weighedefficiently by weak lensing observations; and that most of the FBG's are athigh redshift.Comment: Submitted to ApJ, 15 pages (incl 8 figs, 3 of which are plates). Plate images not included, but are available from ftp://hubble.ifa.hawaii.edu/pub/ger/ms1054/ms1054_fig[1,3,5].ps.