Losing Weight: A KECK Spectroscopic Survey of the Massive Cluster of Galaxies RX J1347-1145

We present a sample of 47 spectroscopically confirmed members of RX J1347-1145, 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 and red spectral indices, with spectra similar to old local ellipticals. Using all 47 cluster members, we derive a mean redshift of 0.4509\pm 0.003, and a velocity dispersion of 910\pm130 km/sec, which corresponds to a virial mass of 4.4 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 deduced from 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 analyses of Allen (2000), Ettori, Allen & Fabian (2001) and Allen, Schmidt & Fabian (2002). Furthermore, the dynamical mass is significantly smaller than that derived from weak lensing (Fischer & Tyson 1997) and from strong lensing (Sahu et al. 1998). We propose that these various discrepant mass estimates may be understood if RX J1347-1145 is the product of two clusters caught in the act of merging in a direction perpendicular to the line of sight, although there is no evidence from the galaxy redshift distribution supporting this hypothesis. Even with this hypothesis, a significant part of the extremely high X-ray luminosity must still arise from non-virialized, presumably shocked, gas. Finally, we report the serendipitous discovery of a lensed background galaxy at z=4.083 which will put strong constraints on the lensing mass determination once its counter-image is securely identified.