2018MNRAS.476..852J

Cold (T 10⁴ K) gas is very commonly found in both galactic and cluster haloes. There is no clear consensus on its origin. Such gas could be uplifted from the central galaxy by galactic or AGN winds. Alternatively, it could form in situ by thermal instability. Fragmentation into a multiphase medium has previously been shown in hydrodynamic simulations to take place once t_cool/t_ff, the ratio of the cooling time to the free-fall time, falls below a threshold value. Here, we use 3D plane-parallel MHD simulations to investigate the influence of magnetic fields. We find that because magnetic tension suppresses buoyant oscillations of condensing gas, it destabilizes all scales below l_A^cool v_A t_cool, enhancing thermal instability. This effect is surprisingly independent of magnetic field orientation or cooling curve shape, and sets in even at very low magnetic field strengths. Magnetic fields critically modify both the amplitude and morphology of thermal instability, with δρ/ρ ∝ β^–1/2, where β is the ratio of thermal to magnetic pressure. In galactic haloes, magnetic fields can render gas throughout the entire halo thermally unstable, and may be an attractive explanation for the ubiquity of cold gas, even in the haloes of passive, quenched galaxies.