Determine the exact cold-CGM density-PDF shape

Characterize the exact shape of the cold circumgalactic-medium density probability distribution, including the effects of unresolved instabilities at the cold–hot gas interface and the microphysics of thermal conduction, viscosity, and magnetic fields.

Background

The paper finds that simulated cold circumgalactic-medium density distributions are approximately skewed lognormal but substantially narrower than the distributions inferred from observed HeII/Lyα ratios. The authors suggest that high-density cold substructures may form through instabilities at the interface between cold and hot gas, including condensation, compression, and fragmentation, but note that these processes are likely under-resolved in the simulations.

The characteristic scale and resulting density distribution depend sensitively on unresolved microphysical processes such as thermal conduction, viscosity, and magnetic fields. Consequently, the exact form of the cold-CGM density PDF remains uncertain, and resolving it is important for explaining the observed Lyα surface-brightness distribution and clumping factors of quasar nebulae.