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

Abstract

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 ∼ 1400~h₇₀^-1 kpc (approximately the virial radius of the cluster). The overall surface brightness profile is well fitted over three orders of magnitude with a simple β-model with a core radius of 0.95 arcmin and β = 0.63. The projected temperature profile declines steeply with radius (by ∼ 50%), and is well described by a polytrope with parameters t₀ = 8.09 keV and γ=1.15. The temperature map shows a pronounced peak in the central arcminute, where the temperature rises by a factor of two (from ∼ 5 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 ∼ 80~h₅₀^-1 kpc, although the agreement between X-ray and weak lensing mass measurements at larger radius (r ∼ 400~h₅₀^-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 different mass estimates of this cluster can be reconciled, at least with standard modelling assumptions.