Active galactic nuclei winds as the origin of the H$_2$ emission excess in nearby galaxies

TL;DR

This study shows that excess H2 emission in nearby galaxies, especially those with active galactic nuclei, is likely caused by shocks from AGN-driven outflows, providing a new way to identify molecular winds.

Contribution

It demonstrates that H2 emission excess correlates strongly with [OI] emission and kinematics, indicating AGN-driven shocks as the origin, and proposes a new galaxy selection method for molecular outflows.

Findings

H2 excess correlates with [OI] emission and kinematics.

AGN-driven outflows induce shocks producing H2 emission.

New indirect method to detect neutral and molecular winds.

Abstract

In most galaxies, the fluxes of rotational H2 lines strongly correlate with star formation diagnostics (such as polycyclic aromatic hydrocarbons, PAH), suggesting that H2 emission from warm molecular gas is a minor byproduct of star formation. We analyse the optical properties of a sample of 309 nearby galaxies derived from a parent sample of 2,015 objects observed with the Spitzer Space Telescope. We find a correlation between the [OI]6300 emission-line flux and kinematics and the H2 S(3)9.665um/PAH11.3um. The [OI]6300 kinematics in Active Galactic Nuclei (AGN) can not be explained only by gas motions due to the gravitational potential of their host galaxies, suggesting that AGN driven outflows are important to the observed kinematics. While H2 excess also correlates with the fluxes and kinematics of ionized gas (probed by [OIII]), the correlation with [OI] is much stronger, suggesting that H2 and [OI] emission probe the same phase or tightly coupled phases of the wind. We conclude that the excess of H2 emission seen in AGN is produced by shocks due to AGN driven outflows and in the same clouds that produce the [OI] emission. Our results provide an indirect detection of neutral and molecular winds and suggest a new way to select galaxies that likely host molecular outflows. Further ground- and space-based spatially resolved observations of different phases of the molecular gas (cold, warm and hot) are necessary to test our new selection method.