PRUSSIC

III. ALMA and NOEMA survey of dense gas in high-redshift star-forming galaxies

¹ Leiden Observatory, Leiden University P.O. Box 9513 2300 RA Leiden, The Netherlands
² Faculty of Electrical Engineering, Mathematics and Computer Science, Delft University of Technology Mekelweg 4 2628 CD Delft, The Netherlands
³ SRON – Netherlands Institute for Space Research Niels Bohrweg 4 2333 CA Leiden, The Netherlands
⁴ Institut für Astrophysik, Universität zu Köln Zülpicher Straße 77 D-50937 Köln, Germany
⁵ European Southern Observatory (ESO) Karl-Schwarzschild-Straße 2 Garching 85748, Germany
⁶ Cosmic Dawn Center (DAWN) København, Denmark
⁷ DTU–Space, Technical University of Denmark Elektrovej 327 2800 Kgs. Lyngby, Denmark
⁸ Department of Physics and Astronomy, University College London Gower Street London WC1E 6BT, United Kingdom
⁹ Transdisciplinary Research Area (TRA) Matter /Argelander-Institut für Astronomie, University of Bonn Bonn, Germany
¹⁰ Max–Planck Institut für Astronomie Königstuhl 17 69117 Heidelberg, Germany
¹¹ Department of Space, Earth and Environment, Chalmers University of Technology SE-412 96 Gothenburg, Sweden

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 13 August 2025
Accepted: 4 November 2025

Abstract

Characterising the relationship between dense gas and star formation is critical for understanding the assembly of galaxies throughout cosmic history. However, due to the faintness of standard dense-gas tracers – HCN, HCO⁺, and HNC – dense gas in high-redshift galaxies remains largely unexplored. We present ALMA and NOEMA observations targeting HCN/HCO⁺/HNC (3–2) and (4–3) emission lines in 11 (mostly) gravitationally lensed dusty star-forming galaxies (DSFGs) at redshift z = 1.6 − 3.2. We detect at least one line in 10 out of 11 galaxies. Altogether, we detect 34 dense-gas transitions, more than quadrupling the number of extant high-redshift detections. Additionally, in two targets, we detect lower-abundance CO isotopologues ¹³CO and C¹⁸O, as well as CN emission. We derive excitation coefficients for HCN, HCO⁺, and HNC in DSFGs, finding them to be systematically higher than those in nearby luminous infrared galaxies. Assuming the canonical dense-mass conversion factor (α_HCN = 10), we find that DSFGs have shorter dense-gas depletion times (median 23 Myr) than nearby galaxies (≈60 Myr), with a star-forming efficiency per free-fall time of 1−2%, a factor of a few higher than in local galaxies. We find a wide range of dense-gas fractions, with HCN/CO ratios ranging between 0.01 and 0.15. Finally, we put the first constraints on the redshift evolution of the cosmic dense-gas density, which increases by a factor of 7 ± 4 between z = 0 and z = 2.5, consistent with the evolution of the cosmic molecular-gas density.