Direct observations of the atomic-molecular phase transition in the Milky Way's nuclear wind

Hundreds of high-velocity atomic gas clouds exist above and below the Galactic Centre, with some containing a molecular component. However, the origin of these clouds in the Milky Way's wind is unclear. This paper presents new high-resolution MeerKAT observations of three atomic gas clouds and studies the relationship between the atomic and molecular phases at ∼1 pc scales. The clouds' atomic hydrogen column densities, NH i, are less than a few × 1020 cm-2, but the two clouds closest to the Galactic Centre none the less have detectable CO emission. This implies the presence of H2 at levels of NH i at least a factor of ten lower than in the typical Galactic interstellar medium. For the cloud closest to the Galactic Centre, detectable CO coexists across the entire range of H i column densities. In contrast, for the intermediate cloud, detectable CO is heavily biased towards the highest values of NH i. The cloud most distant from the Galactic Centre has no detectable CO at similar NH i values. Moreover, we find that the two clouds with detectable CO are too molecule-rich to be in chemical equilibrium, given the depths of their atomic shielding layers, which suggests a scenario whereby these clouds consist of pre-existing molecular gas from the disc that the Galactic wind has swept up, and that is dissociating into atomic hydrogen as it flows away from the Galaxy. We estimate that entrained molecular material of this type has a ∼few - 10 Myr lifetime before photodissociating.