PRUSSIC

I. A JVLA survey of HCN, HCO⁺, and HNC (1–0) emission in z ∼ 3 dusty galaxies: Low dense-gas fractions in high-redshift star-forming galaxies^⋆

¹ Faculty of Electrical Engineering, Mathematics and Computer Science, Delft University of Technology, Delft, The Netherlands
² Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA, Leiden, The Netherlands
e-mail: m.rybak@tudelft.nl
³ Cosmic Dawn Center (DAWN), København, Denmark
⁴ DTU-Space, Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark
⁵ Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK
⁶ I. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany
⁷ Centro de Astrobiología (CAB), CSIC-INTA, Cra. de Ajalvir Km. 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁸ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany
⁹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹⁰ SKA Observatory, Lower Withington Maccleseld, Cheshire, SK11 9FT, UK

Received: 28 April 2022
Accepted: 18 July 2022

Abstract

Dusty star-forming galaxies (DSFGs) at redshift z ≥ 1 are among the most vigorously star-forming galaxies in the Universe. However, their dense (≥10⁵ cm⁻³) gas phase – typically traced by HCN(1–0) – remains almost entirely unexplored: only two DSFGs have been detected in HCN(1–0) to date. We present the results of a Karl G. Jansky Very Large Array survey of the J = 1–0 transition of HCN, HCO⁺, and HNC(1–0) in six strongly lensed DSFGs at z = 2.5 − 3.3, effectively doubling the number of DSFGs with deep observations of these lines. We detect HCN(1–0) emission in one source (J1202+5354, 4.6 σ), with a tentative HCO⁺(1–0) detection in another (J1609+6045, 3.3σ). Spectral stacking yields strict upper limits on the HCN/FIR (≤3.6 × 10⁻⁴) and HCN/CO(1–0) ratios (≤0.045). The inferred HCN/FIR ratios (a proxy for the star-formation efficiency) are consistent with those in z ∼ 0 far-infrared-luminous starbursts. However, the HCN/CO ratios – a proxy for the dense-gas fraction – are a factor of a few lower than suggested by the two previous DSFG detections. Our results imply that most DSFGs have low dense-gas fractions. A comparison with theoretical models of star-forming galaxies indicates that the bulk of gas in DSFGs is at lower densities (≈10² cm⁻³), similar to ‘normal’ star-forming galaxies, rather than ultraluminous starbursts.