2020A&A...634L..14C

We present new observations, carried out withIRAM NOEMA, of the atomic neutral carbon transitions [CI](³P₁-³P₀) at 492 GHz and [CI](³P₂-³P₁) at 809 GHz of GN20, a well-studied star-bursting galaxy at z=4.05. The high luminosity line ratio [CI](³P₂-³P₁)/[CI](³P₁-³P₀) implies an excitation temperature of 48⁺¹⁴_–948-9+14 48^{+14}_{-9} K, which is significantly higher than the apparent dust temperature of T_d=33±2K (β=1.9) derived under the common assumption of an optically thin far-infrared dust emission, but fully consistent with T_d=52±5K of a general opacity model where the optical depth (τ) reaches unity at a wavelength of λ₀=170±23µm. Moreover, the general opacity solution returns a factor of ∼2x 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.