PRUSSIC

Astronomy and Astrophysics, vol. 706, pp. A69

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 13 CO and C 18 O, 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.

Authors 9

1. Matus Rybak Aachen Leiden Observatory SRON Netherlands Institute for Space Research

   Leiden University · Leiden Observatory · Space Research Organisation Netherlands · Delft University of Technology

   Affiliation as printed

   Faculty of Electrical Engineering, Mathematics and Computer Science, Delft University of Technology

   Leiden Observatory, Leiden University

   SRON – Netherlands Institute for Space Research

2. G. Sallaberry Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University

3. Jacqueline Hodge Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University

4. Dominik A. Riechers

   University of Cologne

   Affiliation as printed

   Institut für Astrophysik, Universität zu Köln

5. Nikki N. Geesink Aachen Leiden Observatory

   Leiden University · European Southern Observatory · European Southern Observatory · Leiden Observatory

   Affiliation as printed

   European Southern Observatory (ESO)

   Leiden Observatory, Leiden University

6. T. R. Greve

   DTU Space · University College London · Technical University of Denmark

   Affiliation as printed

   Cosmic Dawn Center (DAWN)

   DTU–Space, Technical University of Denmark

   Department of Physics and Astronomy, University College London

7. S. Viti Aachen Leiden Observatory

   Leiden University · University of Bonn · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University

   Transdisciplinary Research Area (TRA) Matter /Argelander-Institut für Astronomie, University of Bonn

8. P. P. van der Werf Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University

9. C. Yang

   Chalmers University of Technology

   Affiliation as printed

   Department of Space, Earth and Environment, Chalmers University of Technology