The effects of magnetic fields on the growth of thermal instabilities in cooling flows

Abstract

We have examined the effects of heat conduction and magnetic fields on the growth of thermal instabilities in cooling flows using a time-dependent hydrodynamics code. We find that for magnetic fields strengths of ~ 1 (mu}G (similar to that which may exist in intracluster gas), magnetic pressure forces can completely suppress shocks from forming in thermally unstable entropy perturbations with initial length scales as large as 20 kpc, even for initial amplitudes as great as 60%. The suppression of shock formation in cooling condensations significantly reduces the predicted luminosity of optical and ultraviolet emission lines produced as thermally unstable gas cools and decouples from a cooling flow. We find that perturbations with initial amplitudes of 50% and initial magnetic field strengths of 1 μG cool to 10^4^ K on a time scale which is only 22% of the initial instantaneous cooling time. Nonlinear perturbations can thus condense out of cooling flows on a time scale substantially less than the time required for linear perturbations and produce significant mass deposition of cold gas while the accreting intracluster gas is still at large radii. Our models with heat conduction indicate that in order for entropy perturbations to condense out of cooling flows the magnetic fields must be sufficiently tangled to reduce the Spitzer conductivity by a factor of ~100. We discuss some observational consequences of cooling condensations in cooling flows, in particular, the Faraday rotation that can be generated by cold, dense, high magnetic pressure clouds lying along the line of sight to cluster radio sources.