A STATISTICAL COMPARISON OF CLUSTER MASS ESTIMATES FROM OPTICAL X-RAYOBSERVATIONS AND GRAVITATIONAL LENSING/

We present a statistical comparison of three different estimates of cl uster mass, namely, the dynamical masses obtained from the velocity di spersion of optical galaxies, the X-ray masses measured from the tempe rature of X-ray-emitting gas under the assumption of isothermal hydros tatic equilibrium, and the gravitational lensing masses derived from t he strong/weak distortions of background galaxy images. Using a sample of 29 lensing clusters available in literature, we show that the dyna mical masses are in agreement with the gravitational lensing masses wh ile the X-ray method has systematically underestimated cluster masses by a factor of 2-3 as compared with the others. These results imply th at galaxies indeed trace the gravitational potential of their clusters and that there is no bias between the velocities of the dark matter p articles and the galaxies in clusters. The X-ray cluster mass discrepa ncy probably arises from the simplification in the models for the X-ra y gas distribution and dynamical evolution.