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 elliptical galaxies. Using all 47 cluster members, we derive a mean redshift of = 0.4509 ± 0.003 and a velocity dispersion of 910 ± 130 km s^-1, which corresponds to a virial mass of 4.4 × 10¹⁴ h^-1 M_☉ with a 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 and coworkers in 1997), but not consistent with the more recent X-ray analyses of Allen in 2000, Ettori, Allen, and Fabian in 2001, and Allen, Schmidt, and Fabian in 2002. Furthermore, the dynamical mass is significantly smaller than that derived from weak lensing (Fischer and Tyson in 1997) and from strong lensing (Sahu and coworkers in 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 nonvirialized, presumably shocked, gas. Finally, we report the serendipitous discovery of a lensed background galaxy at z = 4.083 that will put strong constraints on the lensing mass determination once its counterimage is securely identified.