Dense-gas tracers and carbon isotopes in five 2.5 < z < 4 lensed dusty star-forming galaxies from the SPT SMG sample

¹ Aix-Marseille Univ., CNRS, CNES, LAM, Marseille, France
e-mail: matthieu.bethermin@lam.fr
² Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK
³ European Southern Observatory, Karl Schwarzschild Straße 2, 85748 Garching, Germany
⁴ Department of Astronomy and Department of Physics, University of Illinois, 1002 West Green St., Urbana, IL, 61801, USA
⁵ Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército 441, Santiago, Chile
⁶ Dalhousie University, Halifax, Nova Scotia, Canada
⁷ Department of Astronomy, University of Florida, Gainesville, FL, 32611, USA
⁸ Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY, 10010, USA
⁹ Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA, 94305, USA
¹⁰ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ, 85721, USA
¹¹ University of Florida Informatics Institute, 432 Newell Drive, CISE Bldg E251, Gainesville, FL, 32611, USA
¹² Cosmic Dawn Centre (DAWN), University of Copenhagen, Julian Maries vej 30, 2100 Copenhagen, Denmark
¹³ Department of Astronomy, University of Texas at Austin, 2515 Speedway Stop C1400, Austin, TX, 78712, USA
¹⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA, 02138, USA
¹⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 23 March 2018
Accepted: 9 October 2018

Abstract

The origin of the high star formation rates (SFR) observed in high-redshift dusty star-forming galaxies is still unknown. Large fractions of dense molecular gas might provide part of the explanation, but there are few observational constraints on the amount of dense gas in high-redshift systems dominated by star formation. In this paper, we present the results of our Atacama large millimeter array (ALMA) program targeting dense-gas tracers (HCN(5-4), HCO⁺(5-4), and HNC(5-4)) in five strongly lensed galaxies from the South Pole Telescope (SPT) submillimeter galaxy sample. We detected two of these lines (S/N > 5) in SPT-125-47 at z = 2.51 and tentatively detected all three (S/N ∼ 3) in SPT0551-50 at z = 3.16. Since a significant fraction of our target lines is not detected, we developed a statistical method to derive unbiased mean properties of our sample taking into account both detections and non-detections. On average, the HCN(5-4) and HCO⁺(5-4) luminosities of our sources are a factor of ∼1.7 fainter than expected, based on the local L′_HCN(5-4) − L_IR relation, but this offset corresponds to only ∼2σ if we consider sample variance. We find that both the HCO⁺/HCN and HNC/HCN flux ratios are compatible with unity. The first ratio is expected for photo-dominated regions (PDRs) while the second is consistent with PDRs or X-ray dominated regions (XDRs) and/or mid-infrared (IR) pumping of HNC. Our sources are at the high end of the local relation between the star formation efficiency, determined using the L_IR/[CI] and L_IR/CO ratios, and the dense-gas fraction, estimated using the HCN/[CI] and HCN/CO ratios. Finally, in SPT0125-47, which has the highest signal-to-noise ratio, we found that the velocity profiles of the lines tracing dense (HCN, HCO⁺) and lower-density (CO, [CI]) molecular gas are similar. In addition to these lines, we obtained one robust and one tentative detection of ¹³CO(4-3) and found an average I _¹²CO(4-3)/I_¹³CO(4-3) flux ratio of 26.1_−3.5^+4.5, indicating a young but not pristine interstellar medium. We argue that the combination of large and slightly enriched gas reservoirs and high dense-gas fractions could explain the prodigious star formation in these systems.