The effects of magnetic fields in cold clouds in cooling flows

Abstract

Large masses of absorbing material are inferred to exist in cooling flows in clusters of galaxies from the excess X-ray absorption in the spectra of some X-ray clusters. The absorbing material is probably in the form of cold clouds pressure-confined by the surrounding, hot, X-ray emitting gas. The cold clouds could remain relatively static until they are destroyed by evaporation or ablation, or give rise to star formation. If the final fate of the clouds is stars, the IMF of the stars formed over the whole cooling flow region ($r \sim 100$ kpc) should be biased to low masses, to avoid a very luminous, blue halo for the central galaxy of the cooling flow. However, there is evidence for bright star formation in the innermost (r < 10 kpc) regions of some cooling flows, and, therefore, the biasing of the IMF towards low masses should not occur or be less important at smaller radii. The consideration of magnetic fields may shed light on these two points. If magnetic fields are present, the magnetic critical mass should be considered, besides the Jeans mass, in establishing a natural mass scale for star formation. When this new mass scale is taken into account, we obtain the right variation of the biasing of the IMF with the radius in addition to inhibition of high-mass star formation at large radii. We also demonstrate that magnetic reconnection is a efficient than ambipolar diffusion in removing magnetic fields in cold clouds.