PRUSSIC

Astronomy and Astrophysics, vol. 667, pp. A70

Abstract

Dusty star-forming galaxies (DSFGs) at redshiftz ≥ 1 are among the most vigorously star-forming galaxies in the Universe. However, their dense (≥105cm−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 aKarl G. JanskyVery Large Array survey of theJ = 1–0 transition of HCN, HCO+, and HNC(1–0) in six strongly lensed DSFGs atz = 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, 4.6σ), 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 inz ∼ 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 (≈102cm−3), similar to ‘normal’ star-forming galaxies, rather than ultraluminous starbursts.

Authors 9

1. Matus Rybak Aachen Leiden Observatory

   Leiden University · Leiden Observatory · Delft University of Technology

   Affiliation as printed

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

   Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA, Leiden, The Netherlands

2. Jacqueline Hodge Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA, Leiden, The Netherlands

3. T. R. Greve

   DTU Space · University College London · Technical University of Denmark

   Affiliation as printed

   Cosmic Dawn Center (DAWN), København, Denmark

   DTU-Space, Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark

   Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK

4. Dominik A. Riechers

   University of Cologne

   Affiliation as printed

   I. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany

5. Isabella Lamperti

   Centro de Astrobiología

   Affiliation as printed

   Centro de Astrobiología (CAB), CSIC-INTA, Cra. de Ajalvir Km. 4, 28850 Torrejón de Ardoz, Madrid, Spain

6. Joshiwa van Marrewijk Aachen Leiden Observatory

   Leiden University · European Southern Observatory · Leiden Observatory

   Affiliation as printed

   European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany

   Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA, Leiden, The Netherlands

7. Fabian Walter

   Max Planck Institute for Astronomy

   Affiliation as printed

   Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany

8. Jeff Wagg

   SKA Observatory

   Affiliation as printed

   SKA Observatory, Lower Withington Maccleseld, Cheshire, SK11 9FT, UK

9. Paul P. van der Werf Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA, Leiden, The Netherlands