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

Isabella Cortzen*, Georgios E. Magdis, Francesco Valentino, Emanuele Daddi, Daizhong Liu, Dimitra Rigopoulou, Mark Sargent, Dominik Riechers, Diane Cormier, Jacqueline A. Hodge, Fabian Walter, David Elbaz, Matthieu Béthermin, Thomas R. Greve, Vasily Kokorev, Sune Toft

*Corresponding author for this work

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We present new observations, carried out with IRAM NOEMA, of the atomic neutral carbon transitions [CI](1-0) at 492 GHz and [CI](2-1) at 809 GHz of GN20, a well-studied star-bursting galaxy at z=4.05. The high luminosity line ratio [CI](2-1)/[CI](1-0) 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.
Original languageEnglish
Article numberL14
JournalAstronomy & Astrophysics
Number of pages6
Publication statusPublished - 2020


  • Galaxies: evolution
  • Galaxies: high-redshift galaxies: ISM
  • Galaxies: starburst

Cite this

Cortzen, I., Magdis, G. E., Valentino, F., Daddi, E., Liu, D., Rigopoulou, D., Sargent, M., Riechers, D., Cormier, D., Hodge, J. A., Walter, F., Elbaz, D., Béthermin, M., Greve, T. R., Kokorev, V., & Toft, S. (2020). Deceptively cold dust in the massive starburst galaxy GN20 at z∼4. Astronomy & Astrophysics, 634, [L14].