Letter to the Editor

Deceptively cold dust in the massive starburst galaxy GN20 at z ∼ 4

¹ Cosmic Dawn Center (DAWN), Lyngbyvej 2, 2100 Copenhagen Ø, Denmark
² Niels Bohr Institute, University of Copenhagen, Blegdamsvej 17, 2100 Copenhagen, Denmark
e-mail: cortzen@nbi.ku.dk
³ DTU-Space, Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark
⁴ CEA, IRFU, DAp, AIM, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, CNRS, 91191 Gif-sur-Yvette, France
⁵ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
⁷ Astronomy Centre, Department of Physics and Astronomy, University of Sussex, Brighton BN1 9QH, UK
⁸ Department of Astronomy, Cornell University, Space Sciences Building, Ithaca, NY 14853, USA
⁹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁰ Aix Marseille Univ., Centre National de la Recherche Scientifique, Laboratoire d’Astrophysique de Marseille, Marseille, France
¹¹ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK

Received: 28 November 2019
Accepted: 4 February 2020

Abstract

We present new observations, carried out with IRAM NOEMA, of the atomic neutral carbon transitions [C I](³P₁–³P₀) at 492 GHz and [C I](³P₂–³P₁) at 809 GHz of GN20, a well-studied star-bursting galaxy at z = 4.05. The high luminosity line ratio [C I](³P₂–³P₁) /[C I](³P₁–³P₀) implies an excitation temperature of 48⁺¹⁴₋₉ K, which is significantly higher than the apparent dust temperature of T_d = 33 ± 2 K (β = 1.9) derived under the common assumption of an optically thin far-infrared dust emission, but fully consistent with T_d = 52 ± 5 K of a general opacity model where the optical depth (τ) reaches unity at a wavelength of λ₀ = 170 ± 23 μm. Moreover, the general opacity solution returns a factor of ∼2× lower dust mass and, hence, a lower molecular gas mass for a fixed gas-to-dust ratio, than with the optically thin dust model. The derived properties of GN20 thus provide an appealing solution to the puzzling discovery of starbursts appearing colder than main-sequence galaxies above z >  2.5, in addition to a lower dust-to-stellar mass ratio that approaches the physical value predicted for starburst galaxies.