The impact of magnetic fields on thermal instability
  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.
