Deceptively cold dust in the massive starburst galaxy GN20 at $z\sim 4$

We present new observations, carried out with IRAM NOEMA, of the atomic neutral carbon transitions CI at 492 GHz and CI at 809 GHz of GN20, a well-studied star-bursting galaxy at $z=4.05$. The high luminosity line ratio CI/CI implies an excitation temperature of $48_{-9}^{+14}$ K, which is significantly higher than the apparent dust temperature of $T_{\rm d}=33\pm2$K($\beta=1.9$) derived under the common assumption of an optically thin far-infrared dust emission, but fully consistent with $T_{\mathrm{d}}=52\pm 5$ K of a general opacity model where the optical depth ($\tau$) reaches unity at a wavelength of ${\lambda }_0=170\pm 23$ $\mu$m. Moreover, the general opacity solution returns a factor of $\sim 2\times$ 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.