Issue |
A&A
Volume 672, April 2023
|
|
---|---|---|
Article Number | A96 | |
Number of page(s) | 13 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/202244317 | |
Published online | 05 April 2023 |
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
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 Rgal ≲ 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 (Σstar), the intensity-weighted CO(2–1) (⟨ICO⟩), and the star formation rate surface density (ΣSFR). We find no apparent correlation with ΣSFR, but positive correlations with the other two parameters, which are stronger in the case of ⟨ICO⟩. The HCN/CO–⟨ICO⟩ relations show ≲0.3 dex galaxy-to-galaxy offsets, with HCN(3–2)/CO(2–1)–⟨ICO⟩ being ∼2 times steeper than HCN(1–0)/CO(2–1). In contrast, the HCN(3–2)/HCN(1–0)–⟨ICO⟩ 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).
Key words: galaxies: star formation / radio lines: galaxies / radio lines: ISM / galaxies: individual: NGC 3351 / galaxies: individual: NGC 3627 / galaxies: individual: NGC 4321
Cosmic Origins Of Life (COOL) Research DAO, coolresearch.io
© The Authors 2023
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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