Evidence from gravitational lensing for a nonthermal pressure support in the cluster of galaxies Abell 2218

Abstract

The central mass distribution of clusters of galaxies can be inferred from gravitationally lensed arcs with known redshifts. For the cluster Abell 2218, this method yields a core mass which is larger by a factor of 2.5 +/- 0.5 than the value deduced from X-ray observations, under the assumptions that the gas is supported by thermal pressure and that the cluster is spherical. We examine various potential causes for this discrepancy and show that a nonthermal pressure support is a plausible explanation. Such a pressure can be provided by strong turbulence and equipartition magnetic fields (similar to 50 mu G) that are tangled on small scales in the cluster core. The turbulent and magnetic pressures do not affect the measured Sunyaev-Zel'dovich effect. Comparable intracluster magnetic fields (similar to 10-100 mu G) have already been detected by Faraday rotation in other clusters. If generic, a small-scale equipartition magnetic field should affect the structure of cooling flows and must be included in X-ray determinations of cluster masses.