Mechanism for high-velocity cold clouds in galactic winds

Determine the physical mechanisms that allow neutral atomic and ionized gas clouds in galactic winds and the circumgalactic medium to be observed moving at velocities of order 200–500 km s−1, discriminating between direct acceleration by outflows (e.g., ram pressure, cosmic-ray pressure, or radiation pressure) and formation via dense-gas precipitation and re-condensation.

Background

Observations of the Milky Way and nearby starburst galaxies reveal multi-phase galactic winds containing neutral atomic and ionized gas clouds that can reach distances of ~1 kpc from the galactic plane and attain velocities of ~200–500 km s−1. Spectroscopic detections of ions such as O VI, N V, C IV, and Si IV further indicate complex kinematics across cold, intermediate, and warm phases in the circumgalactic medium.

Two broad classes of theoretical models have been proposed: (1) direct acceleration of pre-existing cold clouds by ram pressure, cosmic-ray pressure, or radiation pressure from the outflow; and (2) formation of cold clouds by precipitation and re-condensation from the hot wind medium. While radiative, magnetized, and conductive simulations can extend cloud lifetimes, they often struggle to accelerate dense gas to the highest observed speeds, leaving the precise mechanism responsible for the observed fast-moving cold gas unresolved.