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

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 nuclear molecular outflows to be $25.0\\pm2.1$% considering all outflow candidates, or $20\\pm2.4$% 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.