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We present a sample of 47 spectroscopically confirmed members of RXJ1347-1145, the most luminous X-ray cluster of galaxies discovered to date.With two exceptions, all the galaxies in this sample have red B-R colors andred spectral indices, with spectra similar to old local ellipticals. Using all47 cluster members, we derive a mean redshift of 0.4509\pm 0.003, and avelocity dispersion of 910\pm130 km/sec, which corresponds to a virial mass of4.4 x 10^{14} h^{-1} Solar masses with an harmonic radius of 380 h^{-1} kpc.The derived total dynamical mass is marginally consistent with that deducedfrom the cluster's X-ray emission based on the analysis of ROSAT/ASCA images(Schindler et al. 1997), but not consistent with the more recent X-ray analysesof Allen (2000), Ettori, Allen & Fabian (2001) and Allen, Schmidt & Fabian(2002). Furthermore, the dynamical mass is significantly smaller than thatderived from weak lensing (Fischer & Tyson 1997) and from strong lensing (Sahuet al. 1998). We propose that these various discrepant mass estimates may beunderstood if RX J1347-1145 is the product of two clusters caught in the act ofmerging in a direction perpendicular to the line of sight, although there is noevidence from the galaxy redshift distribution supporting this hypothesis. Evenwith this hypothesis, a significant part of the extremely high X-ray luminositymust still arise from non-virialized, presumably shocked, gas. Finally, wereport the serendipitous discovery of a lensed background galaxy at z=4.083which will put strong constraints on the lensing mass determination once itscounter-image is securely identified.Comment: Minor changes to conform to version accepted by Ap

Losing Weight: A Keck Spectroscopic Survey of the Massive Cluster of Galaxies RX J1347−1145