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 $22\%\pm 10\%$ can be accounted for by thermal evaporation of gas from clusters. We have found that evaporation can occur only from the cluster ``surface'', $r\sim r_{vir}$, and not from it's interior. We evaluated particle diffusion through the magnetized ICM for several scenarios of ISM turbulence and found that diffusivity is suppressed by at least a factor of 100 or more, compared to the Spitzer value. Thus, only particles from radii $r\agt 0.9r_{vir}$ can evaporate. Diffusion of particles from inside the cluster, $r\alt 0.9r_{vir}$, takes longer than the Hubble time. This lowers the cluster-averaged fraction of the evaporated hot gas to few percent or less. However, if the missing hot component {\em is indeed} due to evaporation, this strongly constrains the magnetic field structure in the cluster envelope, namely either (i) the gas is completely unmagnetized ($B\alt 10^{-21}$ gauss) in the cluster halo or (ii) the magnetic fields in the ICM are rather homogeneous and non-turbulent.