A dense, solar metallicity ISM in the z = 4.2 dusty star-forming galaxy SPT 0418−47^⋆

¹ European Southern Observatory, Karl Schwarzschild Straße 2, 85748 Garching, Germany
e-mail: cdebreuc@eso.org
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Aix Marseille University, CNRS, LAM, Laboratoire d’Astrophysique de Marseille, Marseille, France
⁴ Department of Astronomy and Physics, Saint Mary’s University, Halifax, NS B3H 3C3, Canada
⁵ Department of Physics and Atmospheric Science, Dalhousie University, Halifax, NS B3H 4R2, Canada
⁶ Departamento de Ciencias Fisicas, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago, Chile
⁷ Millennium Institute of Astrophysics (MAS), Nuncio Monseñor Sotero Sanz 100, Providencia, Santiago, Chile
⁸ Núcleo de Astronomía, Facultad de Ingeniería, Universidad Diego Portales, Av. Ejército 441, Santiago, Chile
⁹ Department of Astronomy, University of Illinois at Urbana-Champaign, 1002 West Green St., Urbana, IL 61801, USA
¹⁰ National Research Council, Herzberg Astronomy and Astrophysics, Victoria, BC, Canada
¹¹ Department of Physics and Astronomy, University of British Columbia, Vancouver, BC V6T 1Z1, Canada
¹² Department of Physics and Astronomy, University of California, Los Angeles, CA 90095-1547, USA
¹³ Department of Astronomy, Cornell University, Ithaca, NY 14853, USA

Received: 24 June 2019
Accepted: 26 September 2019

Abstract

We present a study of six far-infrared fine structure lines in the z = 4.225 lensed dusty star-forming galaxy SPT 0418−47 to probe the physical conditions of its interstellar medium (ISM). In particular, we report Atacama Pathfinder EXperiment (APEX) detections of the [OI] 145 μm and [OIII] 88 μm lines and Atacama Compact Array (ACA) detections of the [NII] 122 and 205 μm lines. The [OI] 145 μm/[CII] 158 μm line ratio is ∼5× higher compared to the average of local galaxies. We interpret this as evidence that the ISM is dominated by photo-dissociation regions with high gas densities. The line ratios, and in particular those of [OIII] 88 μm and [NII] 122 μm imply that the ISM in SPT 0418−47 is already chemically enriched to nearly solar metallicity. While the strong gravitational amplification was required to detect these lines with APEX, larger samples can be observed with the Atacama Large Millimeter/submillimeter Array (ALMA), and should allow observers to determine if the dense, solar metallicity ISM is common among these highly star-forming galaxies.