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

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: stuber@mpia.de
² European Southern Observatory, Karl-Schwarzschild st 2, Garching bei München, Germany
³ Univ Lyon, Univ Lyon1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, 69230 Saint-Genis-Laval, France
⁴ Observatorio Astronómico Nacional (IGN), C/Alfonso XII 3, Madrid 28014, Spain
⁵ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁶ The Observatories of the Carnegie Institution for Science, 813 Santa Barbara Street, Pasadena, CA 91101, USA
⁷ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
⁸ Department of Physics & Astronomy, University of Wyoming, Laramie, WY 82071, USA
⁹ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
¹⁰ Universität Heidelberg, Zentrum für Astronomie, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
¹¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12-14, 69120 Heidelberg, Germany
¹² Department of Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus, OH 43210, USA
¹³ Center for Cosmology and Astroparticle Physics, 191 West Woodruff Avenue, Columbus, OH 43210, USA
¹⁴ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, 9000 Gent, Belgium
¹⁵ 4-183 CCIS, University of Alberta, Edmonton, Alberta, Canada
¹⁶ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching bei München, Germany

Received: 15 April 2021
Accepted: 9 July 2021

Abstract

Central molecular outflows in spiral galaxies are assumed to modulate their host galaxy’s star formation rate (SFR) 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 ∼100 pc. We find correlations between the assigned outflow confidence and stellar mass or global SFR. We determine the frequency of central molecular outflows to be 25 ± 2% considering all outflow candidates, or 20 ± 2% 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 (log₁₀ Ṁ_out [M_⊙ 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.