Where have all the cluster halos gone?

Abstract

A new low-frequency (330 MHz) VLA image of the Perseus cluster confirms the presence of a mini-radio halo with diameter ∼ 430 kpc (H0 = 75 km s-1 Mpc-1) surrounding 3C 84. A careful comparison with the Coma cluster shows that there is no evidence for a similar, very extended halo in Perseus despite the large number of cluster radio galaxies which could power such a halo. These two clusters represent two classes of radio halos which differ by the absence (Coma) or presence (Perseus) of cooling inflows. We argue that smaller halos as in Perseus result from insufficient cluster-wide magnetic fields. A simple model is presented which suggests that cooling flows can suppress the diffusion of turbulently amplified B-fields outward from the cluster core. Such a suppression leads to the development of minihalos which are confined to the cores of cooling flow clusters.