A survey of molecular clouds in the Galactic center's outflow

TL;DR

This survey investigates molecular clouds in the Milky Way's outflow using CO observations, revealing their properties, turbulence, and potential dissociation timescales, thus shedding light on the interaction between hot winds and cold gas.

Contribution

First systematic search for molecular gas in the Milky Way's outflow, characterizing CO clumps and their physical states in the galactic wind environment.

Findings

Detected over 200 CO clumps in 16 HI clouds associated with the outflow.

Most molecular clumps are unbound and highly turbulent, with high internal pressures.

Molecular gas is absent beyond 1 kpc from the Galactic Center, indicating a dissociation timescale of ~3 Myr.

Abstract

The nucleus of the Milky Way is known to drive a large-scale, multiphase galactic outflow, with gas phases ranging from the hot highly-ionized to the cold molecular component. In this work, we present the first systematic search for molecules in the Milky Way wind. We use the Atacama Pathfinder EXperiment (APEX) to observe the 12CO(2-1) emission line in 19 fields centered on previously known high-velocity atomic hydrogen (HI) clouds associated with the outflow. Over 200 CO clumps are detected within 16 different HI clouds. These clumps have typical radii of 1 - 3 parsec, high velocity dispersions of 1 - 6 km/s and molecular gas masses ranging from a few to several hundred solar masses. Molecular clumps in the wind sit on the low-mass end of the mass - size relation of regular molecular clouds, but are far displaced from the mass (or size) - linewidth relation, being generally more turbulent and showing high internal pressures. Nearly 90% of the clumps are gravitationally unbound with virial parameters >> 10 - 100, indicating that these structures are either being disrupted or they must be confined by external pressure from the surrounding hot medium. While the observed properties of CO clumps do not seem to evolve clearly with latitude, we find that molecular gas is not detected in any of the 6 HI clouds with projected distances over 1 kpc from the Galactic Center, suggesting the existence of a maximum timescale of ~ 3 Myr for the dissociation of molecular gas within the wind. Overall, current observations in the Galactic center support a scenario in which a hot wind entrains cold gas clouds from the disk, driving their progressive transformation from molecular to atomic and ultimately ionized gas through stripping, turbulence, and dissociation.