WEAK GRAVITATIONAL LENSING AND X-RAY-ANALYSIS OF ABELL-2163

We report on the detection of dark matter in the cluster of galaxies A bell 2163 using the weak gravitational distortion of background galaxi es and an analysis of the X-ray emission from the cluster. We find tha t while the qualitative distributions of the cluster light and the dar k matter are similar-shallow and extended, with significant substructu re-the X-ray morphology shows a more regular overall appearance, We in terpret the joint lensing and X-ray observations as a signature of a m erger event in the cluster. We present new ROSAT/HRI data and reanalyz e ROSAT/PSPC data, accounting for the effect of a varying background t o determine the best-fit parameters in the beta-model formalism. We co mbine the surface brightness fits with two determinations of the radia l temperature profile using Advanced Satellite for Cosmology and Astro physics and Ginga data to determine the total mass, Although there are slight variations in the total mass determinations introduced by the: uncertainties in the beta-fit, the main contribution to the error ari ses from the uncertainties in the temperature determinations. Even tho ugh the morphologies of the dark matter/light and X-ray gas are somewh at different, we find that the total mass determined from the X-ray an d weak lensing estimates are consistent with each other within the 2 s igma error bars, with the X-ray inferred mass a factor of similar or e qual to 2 larger. However, since the lensing mass estimates are differ ential (the surface density at any point is determined relative to the mean in a control annulus), the shallow, extended nature of the mass profile biases the lensing inferred mass downward. We estimate the cor rection for this effect and find very good agreement between the corre cted lensing and X-ray results. We determine the gas mass fraction in this cluster and find f(g) similar or equal to 0.07 h(-3/2) at all rad ii and a constant mass-to-light ratio of M/L-V = (300 +/- 100) h M/L-. V.