We report on the detection of dark matter in the cluster of galaxies Abell 2163 using the weak gravitational distortion of background galaxies , and an analysis of the X-ray emission from the cluster .
We find that while the qualitative distributions of the cluster light and the dark matter are similar — shallow and extended , with significant substructure — the X-ray morphology shows a more regular overall appearance .
We interpret the joint lensing and X-ray observations as a signature of a merger event in the cluster .
We present new ROSAT/HRI data and reanalyze ROSAT/PSPC data , accounting for the effect of a varying background to determine the best fit parameters in the \beta -model formalism .
We combine the surface brightness fits with two determinations of the radial temperature profile 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 contributor to the error arises from the uncertainties in the temperature determinations .
Even though the morphologies of the dark matter/light and X-ray gas are quite different , we find that the total mass determined from the X-ray and weak lensing estimates are consistent with each other within the 2 \sigma error bars , with the X-ray inferred mass a factor of \simeq 2 larger .
However , as the lensing mass estimates are differential ( 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 downwards .
We estimate the correction for this effect and find very good agreement between the corrected lensing and X-ray results .
We determine the gas mass fraction in this cluster and find f _ { g } \simeq 0.07 h ^ { -3 / 2 } at all radii and a constant mass-to-light ratio of M / L _ { V } = ( 300 \pm 100 ) hM / L _ { \odot V } .