Abell 2218: X-Ray Lensing, Merger, or Both?

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Abstract
Comparison of the high-resolution X-ray image of A2218 obtained with the ROSAT HRI with the optical HST image shows several interesting correlations. The X-ray emission within a 1' radius core is resolved into several components; the central dominant galaxy does not coincide with either of them or with the emission centroid. The major X-ray peak is an elongated feature that lies between the two mass concentrations known from the optical lensing analysis, and coincides with optical arcs at r 20'' from the cD galaxy. We speculate that this may be lensed X-ray emission, for example (but not necessarily) of the same object lensed in the optical. Alternatively, this feature may be a merger shock or a gas trail of an infalling subgroup. Two other X-ray enhancements are close to the two major mass concentrations. Both lensing and a merger are likely. Previous X-ray derivations of the A2218 mass used a β-model fit to the data with angular resolution that blurred the features mentioned above into a broad constant core. As the HRI data show, such a core does not exist. Because of this, under certain assumptions and using only the improved imaging data, the hydrostatic estimate of the projected mass within the lensing radius can in principle be increased by a factor of ~1.4 (and the mass within a sphere of the same radius by a factor of 2.6) compared to previous analyses. However, for a merging cluster, the hydrostatic analysis is generally inapplicable. Most other lensing clusters are more distant than A2218, and obtaining adequate X-ray images and temperature maps of them is even more difficult. Together with the likely overestimation of mass by the lensing analysis (as in the simulations), oversimplification of the gas density and temperature models resulting from inadequate resolution may account for the lensing/X-ray mass discrepancy as suggested for A2218.