The role of magnetic fields in cluster cooling flows

Abstract

Radial inflow and shear of the intracluster magnetic field will produce a dramatic increase in the strength of the field and will cause the field to become more radial within cooling flows. The magnetic pressure becomes comparable to the thermal pressure within a radius of typically 10 kpc. Within this region, reconnection of the magnetic field is efficient, and maintains the field in equipartition with the gas pressure. Buoyancy is ineffective in transporting the magnetic field out of the cooling flow. Reconnection releases the magnetic energy at a rate of 10^42^-10^43^ ergs s^-1^. Part of this energy may accelerate nonthermal, relativistic particles, resulting in diffuse radio emission from the central regions of cooling flows. The largest portion of the reconnection energy is likely to be dissipated in heating the cooling flow gas and may provide a major source of ionization and heating to the emission-line filaments in cooling flows. One dimensional, steady state, numerical models for cooling flows with magnetic fields show that the field has no dramatic effect on the structure of the cooling flow. The primary effect in the inner regions of cooling flows is to effectively increase the heat capacity of the gas by a factor of ~ 2. Also affected by a similar factor are the total gravitational mass derived by applying the hydrostatic equation to the gas pressure, and the accretion rates derived from the gas density and temperature profiles. The most important directly observable effect of the magnetic field in cooling flows is likely to be very strong Faraday rotation of the polarization of radio sources occurring within or behind the cooling flow. The rotation measure should increase rapidly towards the center of the cooling flow (roughly as r^-2^) and should reach values of RM ~ 10^3-4^ rad m^-2^ at projected radii within the equipartition radius. Comparison to several recent observations of the polarization of radio sources associated with the central galaxies in cooling flow clusters suggests that such large rotation measures may be common in cooling flows.