The SWAN view of dense gas in the Whirlpool

A cloud-scale comparison of N₂H⁺, HCO⁺, HNC, and HCN emission in M51

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
² Fakultät für Physik und Astronomie, Universität Heidelberg, Im Neuenheimer Feld 226, 69120 Heidelberg, Germany
³ Observatorio Astronómico Nacional (IGN), C/ Alfonso XII, 3, E-28014 Madrid, Spain
⁴ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁵ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., 02138 Cambridge, MA, USA
⁶ IRAM, 300 rue de la Piscine, F-38406 Saint Martin d’Hères, France
⁷ LUX, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, 75014 Paris, France
⁸ European Southern Observatory, Karl-Schwarzschild 2, 85748 Garching bei Muenchen, Germany
⁹ AURA for ESA, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁰ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08544, USA
¹¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße 1, D-85748 Garching, Germany
¹² Department of Physics & Astronomy, University of Wyoming, Laramie, WY 82071, USA
¹³ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
¹⁴ Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
¹⁵ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
¹⁶ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany
¹⁷ Radcliffe Institute for Advanced Study, Harvard University, 10 Garden St, 02138 Cambridge, MA, USA
¹⁸ Purple Mountain Observatory, Chinese Academy of Sciences, 10 Yuanhua Road, Nanjing 210023, China
¹⁹ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, B-9000 Gent, Belgium
²⁰ Department of Physics, Tamkang University, No.151, Yingzhuan Road, Tamsui District, New Taipei City 251301, Taiwan
²¹ Faculty of Global Interdisciplinary Science and Innovation, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan
²² Sub-department of Astrophysics, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK

^⋆ Corresponding author.

Received: 11 March 2025
Accepted: 18 July 2025

Abstract

Tracing dense molecular gas, the fuel for star formation, is essential for understanding the evolution of molecular clouds and star-formation processes. We compared the emission of HCN (1–0), HNC (1–0), and HCO⁺(1–0) with the emission of N₂H⁺(1–0) at cloud scales (125 pc) across the central 5 × 7 kpc of the Whirlpool galaxy, M51a, from “Surveying the Whirlpool galaxy at Arcseconds with NOEMA” (SWAN). We find that the integrated intensities of HCN, HNC, and HCO⁺ are more steeply correlated with N₂H⁺ emission compared to the bulk molecular gas tracer CO, and we find variations in this relation across the center, molecular ring, northern, and southern disk of M51. Compared to HCN and HNC emission, the HCO⁺ emission follows the N₂H⁺ emission more closely across the environments and physical conditions, such as the surface densities of molecular gas, stellar mass, star-formation rate, dynamical equilibrium pressure, and radius. Under the assumption that N₂H⁺ is a fair tracer of dense gas at these scales, this makes HCO⁺ a more favorable dense gas tracer than HCN within the inner disk of M51. In all environments within our field of view, even when the central 2 kpc are removed, the ratio HCN/CO, which is commonly used to trace average cloud density, is only weakly dependent on molecular gas mass surface density. While ratios of other dense gas lines to CO show a steeper dependence on the surface density of molecular gas, this relation is still shallow in comparison to other nearby star-forming disk galaxies. One reason might be physical conditions in M51, which are different from other normal star-forming galaxies. Increased ionization rates, increased dynamical equilibrium pressure in the central few kiloparsecs, and the impact of the dwarf companion galaxy NGC 5195 are proposed mechanisms that might enhance HCN and HNC emission over HCO⁺ and N₂H⁺ emission at larger-scale environments and cloud scales.