Volume 634, February 2020
|Number of page(s)||6|
|Section||Letters to the Editor|
|Published online||02 March 2020|
Letter to the Editor
Deceptively cold dust in the massive starburst galaxy GN20 at z ∼ 4
Cosmic Dawn Center (DAWN), Lyngbyvej 2, 2100 Copenhagen Ø, Denmark
2 Niels Bohr Institute, University of Copenhagen, Blegdamsvej 17, 2100 Copenhagen, Denmark
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
Accepted: 4 February 2020
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 Td = 33 ± 2 K (β = 1.9) derived under the common assumption of an optically thin far-infrared dust emission, but fully consistent with Td = 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.
Key words: galaxies: evolution / galaxies: high-redshift / galaxies: ISM / galaxies: starburst
© ESO 2020
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