Discovery of a z=0.77 galaxy cluster with multiple, bright, strong-lensing arcs

We report the discovery of a remarkable strong-lensing cluster from the ongoing Red-Sequence Cluster Survey (RCS). RCS 0224-0002, at a spectroscopic redshift of 0.773, lenses two to four separate background sources. These arcs are of relatively high surface brightness and hence amenable to detailed follow-up study. We provide preliminary photometry and spectroscopy of RCS 0224-0002 and discuss in detail the one lensed source for which a redshift has so far been determined. This galaxy, at a redshift of 4.8786, appears to be composed of a star-forming core enveloped in a Lyalpha emitting cloud. Moreover, it shows no measurable velocity structure to a limit of similar to50 km s(-1) over a region likely to be similar to1 kpc in size. The available data are also used to develop an initial lensing model, which shows RCS 0224-0002 to have a mass corresponding to a central velocity dispersion of similar to1000 km s(-1), consistent with the measured optical richness of the cluster. This preliminary model is also used to estimate redshifts for all lensed components visible in our ground-based imaging. RCS 0224-0002 is the highest-redshift cluster to show lensing with such a large number of bright arcs (comparable to the best lensing clusters at much lower redshifts), and the highest-redshift strong-lensing cluster for which both the cluster redshift and at least one source redshift have been established spectroscopically, and as such it provides a powerful opportunity to directly measure the geometry of the universe and galaxy properties at high redshift.