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 \bar { z } = 0.4509 \pm 0.003 , and a velocity dispersion of 910 \pm 130 km s ^ { -1 } , which corresponds to a virial mass of 4.4 \times 10 ^ { 14 } h ^ { -1 } M _ { \odot } 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 .