Imaging and Spectroscopy of Arcs around the Most Luminous X-ray Cluster Rxj 1347.5-1145 1

The cluster RXJ 1347.5-1145, the most luminous cluster in the X-ray wavelengths, was imaged with the newly installed Space Telescope Imaging Spectrograph (STIS) on-board HST. Its relatively high redshift (0.451) and luminosity indicate that this is one of the most massive of all known clusters. The STIS images unambiguously show several arcs in the cluster. The largest two arcs (>5 00 length) are symmetrically situated on opposite sides of the cluster, at a distance of 35 arcsec from the central galaxy. The STIS images also show approximately 100 faint galaxies within the radius of the arcs whose combined luminosity is 4 10 11 L. We also present ground-based spectroscopic observations of the northern arc which show one clear emission line at 6730 A, with a very faint continuum on either side. The emission line is consistent with an identiication as OII] 3727 A, implying a redshift of 0.81 for this arc. The southern arc shows a faint continuum but no emission features. The mass of the lensing cluster, as derived from the gravitational lensing, is 4.2 10 14 M. The resultant mass-to-light ratio of 1000 is higher than what is seen in many clusters but smaller than the value recently derived for somèdark' X-ray clusters (Hattori et al. 1997). The mass derived from the X-ray ux within the radius of the arcs (240 kpc) is 1.3 10 14 M. The ratio of the gravitational to the X-ray mass is 3 which, within the uncertainties, is consistent with the value derived for the whole cluster by Fischer and { 2 { Tyson (1997). Comparison of the mass-to-light ratio and the ratio of the gravitational to X-ray mass implies that at least a major fraction of the dark matter is baryonic.