Can Cluster Evaporation Explain the Missing Thermal Energy in Galaxy Clusters?

Resent observations of a number of galaxy clusters using the Sunyaev-Zel’dovich effect indicate that about 1/3 of baryonic mass is missing from the hot intracluster medium (ICM), which is significantly larger than the fraction of stars and cool gas, which account for only about 10%. Here we address the question whether the remaining 22± 10% can be accounted for by thermal evaporation of gas from clusters. We evaluate particle diffusion through the ICM with tangled, turbulent magnetic fields and find that diffusion is suppressed by a factor ∼ 0.01 from the Spitzer value in the cluster virialized envelope, mostly due to the magnetic mirroring and trapping. Moreover, particles can escape only from a thin shell of thickness of∼ 0.1rvir below the cluster “surface”, r = rvir. Diffusion of particles from inside the cluster, r ∼< 0.9rvir, takes longer than the Hubble time. Therefore, the cluster-averaged fraction of the evaporated hot gas is not exceeding few percent, at most, for typical cluster and ICM parameters. However, if the missing hot component is indeed due to evaporation, this strongly constrains the magnetic field structure in the cluster envelope, namely either (i) the gas is completely unmagnetized (B ≤ 10 gauss) in the cluster halo or (ii) the magnetic fields in the ICM are rather homogeneous and non-turbulent. Subject headings: galaxies: clusters: general — diffusion — magnetic fields