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 [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 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 λ_0=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.