Cortzen, I; Magdis, GE; Valentino, F; Daddi, E; Liu, D; Rigopoulou, D; Sargent, M; ... Toft, S; + view all Cortzen, I; Magdis, GE; Valentino, F; Daddi, E; Liu, D; Rigopoulou, D; Sargent, M; Riechers, D; Cormier, D; Hodge, JA; Walter, F; Elbaz, D; Bethermin, M; Greve, TR; Kokorev, V; Toft, S; - view fewer (2020) Deceptively cold dust in the massive starburst galaxy GN20 at z ∼ 4. Astronomy & Astrophysics , 634 , Article L14. 10.1051/0004-6361/201937217.

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Abstract

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) mplies 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 T_{d} = 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.

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