Molecular outflows in local galaxies: Method comparison and a role of intermittent AGN driving

¹ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
e-mail: lutz@mpe.mpg.de
² NOVA-ASTRON, Oude Hoogeveensedijk, 7991 PD Dwingeloo, The Netherlands
³ Department of Astronomy and Joint Space-Science Institute, University of Maryland, College Park, MD 20742, USA
⁴ Institute of Astronomy and Kavli Institute for Cosmology Cambridge, University of Cambridge, Cambridge CB3 0HA, UK
⁵ Space Telescope Science Institute, Baltimore MD 21218, USA
⁶ Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Observatory, 439 94 Onsala, Sweden
⁷ INAF – Osservatorio Astronomico di Brera, Via Brera 28, 20121 Milano, Italy
⁸ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Blindern 0315 Oslo, Norway
⁹ IRAM, 300 Rue de la Piscine, 38406 Saint Martin D’Hères, Grenoble, France
¹⁰ INAF – Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34143 Trieste, Italy
¹¹ George Mason University, Department of Physics & Astronomy, MS 3F3, 4400 University Drive, Fairfax, VA 22030, USA
¹² Cavendish Laboratory, University of Cambridge, 19 J.J. Thomson Ave., Cambridge CB3 0HE, UK
¹³ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁴ Observatorio Astronómico Nacional (OAN-IGN) – Observatorio de Madrid, Alfonso XII, 28014 Madrid, Spain
¹⁵ Universidad de Alcalá, Departamento de Física y Matemáticas, Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain
¹⁶ Departamento de Astronomía, Universidad de Concepción, Avenida Esteban Iturra s/n, Concepcíon, Chile
¹⁷ School of Physics & Astronomy, Tel Aviv University, Ramat Aviv 69978, Israel
¹⁸ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 28 September 2019
Accepted: 7 November 2019

Abstract

We report new detections and limits from a NOEMA and ALMA CO(1-0) search for molecular outflows in 13 local galaxies with high far-infrared surface brightness, and combine these with local universe CO outflow results from the literature. The CO line ratios and spatial outflow structure of our targets provide some constraints on the conversion steps from observables to physical quantities such as molecular mass outflow rates. Where available, ratios between outflow emission in higher J CO transitions and in CO(1-0) are typically consistent with excitation R_i1 ≲ 1. However, for IRAS 13120−5453, R₃₁ = 2.10 ± 0.29 indicates optically thin CO in the outflow. Like much of the outflow literature, we use α_{CO(1 − 0)} = 0.8, and we present arguments for using C = 1 in deriving molecular mass outflow rates Ṁ_out = CM_outv_out/R_out. We compare the two main methods for molecular outflow detection: CO millimeter interferometry and Herschel OH-based spectroscopic outflow searches. For 26 sources studied with both methods, we find an 80% agreement in detecting v_out ≳ 150 km s⁻¹ outflows, and non-matches can be plausibly ascribed to outflow geometry and signal-to-noise ratio. For a published sample of 12 bright ultraluminous infrared galaxies with detailed OH-based outflow modeling, CO outflows are detected in all but one. Outflow masses, velocities, and sizes for these 11 sources agree well between the two methods, and modest remaining differences may relate to the different but overlapping regions sampled by CO emission and OH absorption. Outflow properties correlate better with active galactic nucleus (AGN) luminosity and with bolometric luminosity than with far-infrared surface brightness. The most massive outflows are found for systems with current AGN activity, but significant outflows in nonAGN systems must relate to star formation or to AGN activity in the recent past. We report scaling relations for the increase of outflow mass, rate, momentum rate, and kinetic power with bolometric luminosity. Short flow times of ∼10⁶ yr and some sources with resolved multiple outflow episodes support a role of intermittent driving, likely by AGNs.