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

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.