Letter to the Editor

Dense gas scaling relations at kiloparsec scales across nearby galaxies with the ALMA ALMOND and IRAM 30 m EMPIRE surveys

¹ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
² European Southern Observatory (ESO), Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
³ Observatorio Astronómico Nacional (IGN), C/ Alfonso XII, 3, E-28014 Madrid, Spain
⁴ Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain
⁵ Department of Astronomy, The Ohio State University, 140 West 18th Ave, Columbus, OH 43210, USA
⁶ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08544, USA
⁷ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁸ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, 75014 Paris, France
⁹ Center for Astrophysics ∣ Harvard & Smithsonian, 60 Garden St., 02138 Cambridge, MA, USA
¹⁰ Max-Planck-Institut für Extraterrestrische Physik (MPE), Giessenbachstr. 1, D-85748 Garching, Germany
¹¹ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
¹² Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
¹³ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany
¹⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹⁵ Elizabeth S. and Richard M. Cashin Fellow at the Radcliffe Institute for Advanced Studies at Harvard University, 10 Garden Street, Cambridge, MA 02138, USA
¹⁶ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12-14, D-69120 Heidelberg, Germany
¹⁷ Purple Mountain Observatory, Chinese Academy of Sciences, 10 Yuanhua Road, Nanjing 210023, China
¹⁸ Department of Physics, Tamkang University, No.151, Yingzhuan Road, Tamsui District, New Taipei City 251301, Taiwan
¹⁹ Sub-department of Astrophysics, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK

^⋆ Corresponding author; lukas.neumann.astro@gmail.com

Received: 28 November 2024
Accepted: 13 December 2024

Abstract

Dense, cold gas is the key ingredient for star formation. Over the last two decades, HCN(1 − 0) emission has been the most accessible dense gas tracer for studying external galaxies. We present new measurements that demonstrate the relationship between dense gas tracers, bulk molecular gas tracers, and star formation in the ALMA ALMOND survey, the largest sample of resolved (1–2 kpc resolution) HCN maps of galaxies in the local Universe (d < 25 Mpc). We measured HCN/CO, a line ratio sensitive to the physical density distribution, and the star formation rate to HCN ratio (SFR/HCN), a proxy for the dense gas star formation efficiency, as a function of molecular gas surface density, stellar mass surface density, and dynamical equilibrium pressure across 31 galaxies (a factor of > 3 more compared to the previously largest such study, EMPIRE). HCN/CO increases (slope of ≈0.5 and scatter of ≈0.2 dex) and SFR/HCN decreases (slope of ≈ − 0.6 and scatter of ≈0.4 dex) with increasing molecular gas surface density, stellar mass surface density, and pressure. Galaxy centres with high stellar mass surface densities show a factor of a few higher HCN/CO and lower SFR/HCN compared to the disc average, but the two environments follow the same average trend. Our results emphasise that molecular gas properties vary systematically with the galactic environment and demonstrate that the scatter in the Gao–Solomon relation (SFR/HCN) has a physical origin.