The impact of magnetic fields on thermal instability

TL;DR

This study uses 3D MHD simulations to show that magnetic fields enhance thermal instability in galactic and cluster halos, potentially explaining the widespread presence of cold gas.

Contribution

It demonstrates that magnetic tension universally destabilizes gas at small scales, regardless of field orientation or cooling curve, highlighting magnetic fields' critical role in thermal instability.

Findings

Magnetic tension destabilizes all scales below a certain length, enhancing thermal instability.

Magnetic fields influence the amplitude and morphology of thermal instability.

Magnetic fields can make entire galactic halos thermally unstable.

Abstract

Cold ($T\sim 10^{4} \ \mathrm{K}$) gas is very commonly found in both galactic and cluster halos. 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 multi-phase medium has previously been shown in hydrodynamic simulations to take place once $t_\mathrm{cool}/t_\mathrm{ff}$, 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 $l_\mathrm{A}^\mathrm{cool} \sim v_\mathrm{A} t_\mathrm{cool}$, 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 $\delta \rho/\rho \propto \beta^{-1/2}$, where $\beta$ is the ratio of thermal to magnetic pressure. In galactic halos, 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 halos of passive, quenched galaxies.