Letter to the Editor

A constant N₂H⁺ (1-0)-to-HCN (1-0) ratio on kiloparsec scales

¹ Observatorio Astronómico Nacional (IGN), C/Alfonso XII 3, 28014 Madrid, Spain
e-mail: mdonaire@oan.es
² Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain
³ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, 75014 Paris, France
⁴ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Vesilinnantie 5, 20014 Turku, Finland
⁵ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
⁶ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁷ Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
⁸ European Southern Observatory (ESO), Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
⁹ Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Strasse 2, 69120 Heidelberg, Germany
¹⁰ Department of Physics & Astronomy, University of Wyoming, Laramie, WY, USA
¹¹ Cardiff Hub for Astrophysics Research & Technology, School of Physics & Astronomy, Cardiff University, Queens Buildings, Cardiff CF24 3AA, UK
¹² Haystack Observatory, Massachusetts Institute of Technology, 99 Millstone Rd., Westford, MA 01886, USA
¹³ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany
¹⁴ Department of Astronomy, The Ohio State University, 140 West 18th Ave, Columbus, OH 43210, USA
¹⁵ Department of Physics, Tamkang University, No. 151, Yingzhuan Road, Tamsui District, New Taipei City 251301, Taiwan
¹⁶ Institut de Radioastronomie Millimétrique (IRAM), 300 Rue de la Piscine, 38406 Saint Martin d’Hères, France
¹⁷ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁸ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹⁹ Sub-department of Astrophysics, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK

Received: 30 May 2023
Accepted: 31 July 2023

Abstract

Nitrogen hydrides such as NH₃ and N₂H⁺ are widely used by Galactic observers to trace the cold dense regions of the interstellar medium. In external galaxies, because of limited sensitivity, HCN has become the most common tracer of dense gas over large parts of galaxies. We provide the first systematic measurements of N₂H⁺ (1-0) across different environments of an external spiral galaxy, NGC 6946. We find a strong correlation (r > 0.98, p < 0.01) between the HCN (1-0) and N₂H⁺ (1-0) intensities across the inner ∼8 kpc of the galaxy, at kiloparsec scales. This correlation is equally strong between the ratios N₂H⁺ (1-0)/CO (1-0) and HCN (1-0)/CO (1-0), tracers of dense gas fractions (f_dense). We measure an average intensity ratio of N₂H⁺ (1-0)/HCN (1-0) = 0.15 ± 0.02 over our set of five IRAM-30m pointings. These trends are further supported by existing measurements for Galactic and extragalactic sources. This narrow distribution in the average ratio suggests that the observed systematic trends found in kiloparsec-scale extragalactic studies of f_dense and the efficiency of dense gas (SFE_dense) would not change if we employed N₂H⁺ (1-0) as a more direct tracer of dense gas. At kiloparsec scales our results indicate that the HCN (1-0) emission can be used to predict the expected N₂H⁺ (1-0) over those regions. Our results suggest that, even if HCN (1-0) and N₂H⁺ (1-0) trace different density regimes within molecular clouds, subcloud differences average out at kiloparsec scales, yielding the two tracers proportional to each other.