Magnetohydrodynamic Precipitation

TL;DR

This paper explores how magnetic fields influence the formation and organization of cold gas filaments in the hot atmospheres of massive galaxies, providing a theoretical framework to interpret observations and simulations.

Contribution

It introduces a magnetohydrodynamic model explaining the formation of cold filaments in galactic atmospheres and their magnetic properties, complementing numerical simulations.

Findings

Magnetic fields promote cold cloud formation in galactic atmospheres.

Cold gas organizes into magnetized, elongated filaments descending at sub-Keplerian speeds.

The model aids in interpreting observations and simulations of filamentary gas.

Abstract

Circumgalactic gas around massive galaxies generally has a volume-filling component -- an atmosphere -- with a temperature determined by the potential-well depth of the galaxy's halo. If the atmosphere is near hydrostatic equilibrium and is stable to convection, then it can remain nearly homogeneous, as long as it is not too dense. But if its density is great enough, it becomes prone to producing a rain of cold clouds that fall toward the galaxy's center and accrete onto its central black hole. Here we explain how relatively weak magnetic fields enhance a galactic atmosphere's tendency to produce cold clouds and how the cold gas becomes organized into vertically elongated, highly magnetized filaments descending at sub-Keplerian speeds. It is intended to complement recent numerical simulations of the process and to serve as a guide to interpreting both simulations and observations of the filamentary gas in hot galactic atmospheres.