Sub-kiloparsec empirical relations and excitation conditions of HCN and HCO⁺ J = 3–2 in nearby star-forming galaxies

¹ Observatorio Astronómico Nacional (IGN), C/ Alfonso XII, 3, Madrid 28014, Spain
e-mail: axel.garcia@oan.es
² Department of Astronomy, The Ohio State University, 140 West 18th Ave, Columbus, OH 43210, USA
³ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁴ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany
⁵ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁶ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁷ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50157 Firenze, Italy
⁸ Aix Marseille Université, CNRS, CNES, LAM (Laboratoire d’Astrophysique de Marseille), 13388 Marseille, France
⁹ Institut für Theoretische Astrophysik, Zentrum für Astronomie, Universität Heidelberg, Albert-Ueberle-Straße 2, 69120 Heidelberg, Germany
¹⁰ Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071, USA
¹¹ Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Universität Heidelberg, Im Neuenheimer Feld 205, 69120 Heidelberg, Germany
¹² IRAM, 300 rue de la Piscine, 38406 Saint-Martin-d’Hères, France
¹³ Sorbonne Université Observatoire de Paris, Université PSL, École normale supérieure, CNRS, LERMA, 75005 Paris, France
¹⁴ 4-183 CCIS, University of Alberta, Edmonton, AB T6G 2E1, Canada
¹⁵ Center for Astrophysics and Space Sciences, Department of Physics, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA

Received: 21 June 2022
Accepted: 24 November 2022

Abstract

We present new HCN and HCO⁺ (J = 3–2) images of the nearby star-forming galaxies (SFGs) NGC 3351, NGC 3627, and NGC 4321. The observations, obtained with the Morita ALMA Compact Array, have a spatial resolution of ∼290–440 pc and resolve the inner R_gal ≲ 0.6–1 kpc of the targets, as well as the southern bar end of NGC 3627. We complement this data set with publicly available images of lower excitation lines of HCN, HCO⁺, and CO and analyse the behaviour of a representative set of line ratios: HCN(3–2)/HCN(1–0), HCN(3–2)/HCO⁺(3–2), HCN(1–0)/CO(2–1), and HCN(3–2)/CO(2–1). Most of these ratios peak at the galaxy centres and decrease outwards. We compare the HCN and HCO⁺ observations with a grid of one-phase, non-local thermodynamic equilibrium (non-LTE) radiative transfer models and find them compatible with models that predict subthermally excited and optically thick lines. We study the systematic variations of the line ratios across the targets as a function of the stellar surface density (Σ_star), the intensity-weighted CO(2–1) (⟨I_CO⟩), and the star formation rate surface density (Σ_SFR). We find no apparent correlation with Σ_SFR, but positive correlations with the other two parameters, which are stronger in the case of ⟨I_CO⟩. The HCN/CO–⟨I_CO⟩ relations show ≲0.3 dex galaxy-to-galaxy offsets, with HCN(3–2)/CO(2–1)–⟨I_CO⟩ being ∼2 times steeper than HCN(1–0)/CO(2–1). In contrast, the HCN(3–2)/HCN(1–0)–⟨I_CO⟩ relation exhibits a tighter alignment between galaxies. We conclude that the overall behaviour of the line ratios cannot be ascribed to variations in a single excitation parameter (e.g., density or temperature).