1995A&A...303...27K

A mass model of the rich cluster A2218 is presented based on optical and new near-infrared images of arc(let)s and the ROSAT/HRI X-ray map. A lensing model is proposed with two dark matter clumps, centered on the two bright galaxies around which the various arcs are observed. The number of free parameters of the model is reduced to four, namely two for the mass density profile of each clump. The centers, orientations and ellipticities of the mass distribution are fixed to those of the outer isophotes of the central galaxies. The model reproduces the four long arcs present in this cluster. Two of them, close to the most massive clump around the brightest galaxy, are required to be fold images (i.e., two merged images on a critical line), given their position angle with respect to the major axis. A third image for each of the two arcs is then predicted to be on the opposite side of the major axis. Both images are indeed found at the expected positions, with consistent magnitudes, colors and shapes. One of the arcs has a known redshift of 0.7, and the model predicts that the other arc should be at higher redshift, z≃3 (depending on the slope assumed for the density profile). Photometric data in UBrzJK' indicates that a redshift close to 3 is indeed probable for this arc. As in other cluster lenses (MS2137, Abell 370, Cl2236-04), the dark matter halo is found to be sharply peaked on the giant central galaxy. The ROSAT/HRI X-ray map shows a strongly clumped image, poorly correlated with the cD light and the mass model on scales below ∼ 200h₅₀^–1kpc, but similar to the distribution of galaxies and the mass model when smoothed on this scale. This is expected if the cluster was formed by mergers and is not yet relaxed to a stationary equilibrium. The X-ray map is interpreted as due to gas falling along the major axis of the cluster towards a central shock, with a stream of shocked gas moving away from the center along the minor axis. The large degree of central substructure implied helps to account for the discrepancy previously found (in the two massive clusters A2218 & A1689) between the mass derived from the gravitational lenses and that derived from the X-ray profile and temperature distributions, in a model of hydrostatic equilibrium. Most of the discrepancy can probably be explained by bulk motions and acceleration of the gas, and projection effects enhancing the lensing power of the cluster.