Frequency and nature of central molecular outflows in nearby star-forming disk galaxies

TL;DR

This study develops a method to identify and analyze molecular outflows in nearby star-forming galaxies, revealing their frequency, association with AGN and bars, and their limited impact on star formation regulation.

Contribution

The paper introduces a rigorous selection method for molecular outflows using high-resolution CO data, and applies it to a large galaxy sample to quantify outflow frequency and properties.

Findings

Central molecular outflows occur in about 20-25% of galaxies.

Outflows are more common in galaxies with AGN and bars.

Mass loading factors suggest outflows do not significantly quench star formation.

Abstract

Central molecular outflows in spiral galaxies are assumed to modulate their host galaxy's star formation rate by removing gas from the inner region of the galaxy. Outflows consisting of different gas phases appear to be a common feature in local galaxies, yet, little is known about the frequency of molecular outflows in main sequence galaxies in the nearby universe. We develop a rigorous set of selection criteria, which allow the reliable identification of outflows in large samples of galaxies. Our criteria make use of central spectra, position-velocity diagrams and velocity-integrated intensity maps (line-wing maps). We use this method on high-angular resolution CO(2-1) observations from the PHANGS-ALMA survey, which provides observations of the molecular gas for a homogeneous sample of 90 nearby main sequence galaxies at a resolution of ${\sim}100\,$pc. We find correlations between the assigned outflow confidence and stellar mass or global star formation rate (SFR). We determine the frequency of central molecular outflows to be $25\pm2$% considering all outflow candidates, or $20\pm2$% for secure outflows only. Our resulting outflow candidate sample of $16{-}20$ galaxies shows an overall enhanced fraction of active galactic nuclei (AGN) (50%) and bars (89%) compared to the full sample (galaxies with AGN: 24%, with bar: 61%). We extend the trend between mass outflow rates and SFR known for high outflow rates down to lower values ($\log_{10}{\dot{\rm M}_{\rm out}}\,[\mathrm{M}_\odot~\mathrm{yr}^{-1}]<0$). Mass loading factors are of order unity, indicating that these outflows are not efficient in quenching the SFR in main sequence galaxies.