IMAGING AND SPECTROSCOPY OF ARCS AROUND THE MOST LUMINOUS X-RAY-CLUSTER, RX J1347.5-1145

The cluster RX J1347.5-1145, the most luminous cluster in the X-ray wa velengths, was imaged with the newly installed Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope. Its relative ly high redshift (0.451) and luminosity indicate that this is one of t he most massive of all known clusters. The STIS images unambiguously s how several arcs in the cluster. The largest two arcs (>5'' length) ar e symmetrically situated on opposite sides of the cluster, at a distan ce of similar to 35'' from the central galaxy. The STIS images also sh ow approximately 100 faint galaxies within the radius of the arcs whos e combined luminosity is similar to 4 x 10(11) L-.. We also present gr ound-based spectroscopic observations of the northern are that show on e clear emission line at similar to 6730 Angstrom, with a very faint c ontinuum on either side. The emission line is consistent with an ident ification as [O II] lambda 3727, implying a redshift of 0.81 for this are. The southern are shows a faint continuum but no emission features . The surface mass within the radius of the arcs (240 kpc), as derived from the gravitational lensing, is similar to 6.3 x 10(14) M-.. The r esultant mass-to-light ratio of similar to 1200 is higher than what is seen in many clusters but smaller than the value recently derived for some ''dark'' X-ray clusters (Hattori et al.). The total surface mass derived from the X-ray flux within the radius of the arcs is similar to(2.1-6.8) x 10(14) M-., which implies that the ratio of the gravitat ional to the X-ray mass is similar to 1-3. The surface gas mass within this radius is similar to 3.5 x 10(13) M-., which implies that at lea st 6% of the total mass within this region is baryonic.