CON-quest

II. Spatially and spectrally resolved HCN/HCO⁺ line ratios in local luminous and ultraluminous infrared galaxies

¹ Department of Astronomy, The University of Tokyo, 7-3-1, Hongo, Bunkyo, Tokyo 113-0033, Japan
e-mail: nishimura@astron.s.u-tokyo.ac.jp
² Institute of Astronomy, The University of Tokyo, 2-21-1, Osawa, Mitaka, Tokyo 181-0015, Japan
³ National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan
⁴ Department of Space, Earth and Environment, Onsala Space Observatory, Chalmers University of Technology, 439 92 Onsala, Sweden
⁵ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁶ Observatoire de Paris, LERMA, Collège de France, CNRS, PSL University, Sorbonne Université, Paris, France
⁷ Institute of Astrophysics, Foundation for Research and Technology-Hellas (FORTH), Heraklion 70013, Greece
⁸ School of Sciences, European University Cyprus, Diogenes street, Engomi, 1516 Nicosia, Cyprus
⁹ Department of Astronomy, University of Wisconsin-Madison, 5534 Sterling, 475 North Charter Street, Madison, WI 53706, USA
¹⁰ Department of Physics and Astronomy, Macalester College, 1600 Grand Abe, St. Paul, MN 55105, USA
¹¹ Observatorio Astronómico Nacional (OAN-IGN)-Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain
¹² Universidad de Alcalá, Departamento de Física y Matemáticas, Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain
¹³ DTU-Space, National Space Institute, Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark
¹⁴ Cosmic Dawn Center (DAWN), DTU-Space, Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark
¹⁵ Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
¹⁶ Astronomical Science Program, Graduate Institute for Advanced Studies, SOKENDAI, 2-21-1, Osawa, Mitaka, Tokyo 181-1855, Japan
¹⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁸ Astron. Dept., King Abdulaziz University, PO Box 80203 21589 Jeddah, Saudi Arabia
¹⁹ Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA
²⁰ European Southern Observatory, Alonso de Córdova 3107, Vitacura, 763 0355 Santiago, Chile
²¹ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, 763 0355 Santiago, Chile
²² Department of Astronomy, University of Florida, PO Box 112055 Gainesville, FL 32611, USA
²³ Department of Astronomy, University of Virginia, 530 McCormick Road, Charlottesville, VA 22904, USA
²⁴ Núcleo de Astronomía de la Facultad de Ingeniería, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
²⁵ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
²⁶ Institut de Radioastronomie Millimétrique (IRAM), 300 Rue de la Piscine, 38400 Saint-Martin-d’Hères, France
²⁷ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
²⁸ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK

Received: 20 October 2023
Accepted: 23 February 2024

Abstract

Context. Nuclear regions of ultraluminous and luminous infrared galaxies (U/LIRGs) are powered by starbursts and/or active galactic nuclei (AGNs). These regions are often obscured by extremely high columns of gas and dust. Molecular lines in the submillimeter windows have the potential to determine the physical conditions of these compact obscured nuclei (CONs).

Aims. We aim to reveal the distributions of HCN and HCO⁺ emission in local U/LIRGs and investigate whether and how they are related to galaxy properties.

Methods. Using the Atacama Large Millimeter/submillimeter Array (ALMA), we have conducted sensitive observations of the HCN J = 3−2 and HCO⁺J = 3−2 lines toward 23 U/LIRGs in the local Universe (z < 0.07) with a spatial resolution of ∼0.3″ (∼50−400 pc).

Results. We detected both HCN and HCO⁺ in 21 galaxies, only HCN in one galaxy, and neither in one galaxy. The global HCN/HCO⁺ line ratios, averaged over scales of ∼0.5−4 kpc, range from 0.4 to 2.3, with an unweighted mean of 1.1. These line ratios appear to have no systematic trend with bolometric AGN luminosity or star formation rate. The line ratio varies with position and velocity within each galaxy, with an average interquartile range of 0.38 on a spaxel-by-spaxel basis. In eight out of ten galaxies known to have outflows and/or inflows, we found spatially and kinematically symmetric structures of high line ratios. These structures appear as a collimated bicone in two galaxies and as a thin spherical shell in six galaxies.

Conclusions. Non-LTE analysis suggests that the high HCN/HCO⁺ line ratio in outflows is predominantly influenced by the abundance ratio. Chemical model calculations indicate that the enhancement of HCN abundance in outflows is likely due to high-temperature chemistry triggered by shock heating. These results imply that the HCN/HCO⁺ line ratio can aid in identifying the outflow geometry when the shock velocity of the outflows is sufficiently high to heat the gas.