Further evidence for a merger in Abell 2218 from an XMM-Newton observation

The galaxy cluster Abell 2218, at z = 0.171, is well- known for the discrepancy between mass estimates derived from X- ray and strong lensing analyses. With the present XMM- Newton observation, we are able to trace the gas density and temperature profiles out to a radius of similar to 1400 h(-1) (70) kpc ( approximately the virial radius of the cluster). The overall surface brightness profile is well fitted over three orders of magnitude with a simple beta-model with a core radius of 0'.95 and beta = 0.63. The projected temperature profile declines steeply with radius ( by similar to 50%), and is well described by a polytrope with parameters t(0) = 8.09 keV and gamma = 1.15. The temperature map shows a pronounced peak in the central arcminute, where the temperature rises by a factor of two ( from similar to 5 to similar to 10 keV). The mass profile, calculated assuming hydrostatic equilibrium and spherical symmetry, is best fitted with a King approximation to an isothermal sphere, implying a dark matter distribution with a central core, in contrast with the cusped cores found in more obviously relaxed clusters. The X- ray mass is approximately two times less than the strong lensing mass at r similar to 80 h(50)(-1) kpc, although the agreement between X- ray and weak lensing mass measurements at larger radius ( r similar to 400 h(50)(-1) kpc) is slightly better. While the X- ray total mass estimates can vary by 30 per cent depending on the mass model, all measurements are significantly lower than the corresponding total mass from optical measurements. Given the X- ray results indicating considerable disturbance of the intracluster gas, leading to a probable violation of the assumption of hydrostatic equilibrium, and the observed substructure in the optical, suggesting a line- of- sight merger, it is unlikely that the di. erent mass estimates of this cluster can be reconciled, at least with standard modelling assumptions.