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

Abstract : We present new observations, carried out with IRAM NOEMA, of the atomic neutral carbon transitions [C I]($^3$P$_1$$–$$^3$P$_0$) at 492 GHz and [C I]($^3$P$_2$$–$$^3$P$_1$) at 809 GHz of GN20, a well-studied star-bursting galaxy at $z$ = 4.05. The high luminosity line ratio [C I]($^3$P$_2$$–$$^3$P$_1$) /[C I]($^3$P$_1$$–$$^3$P$_0$) implies an excitation temperature of 48$^{+14}_{−9}$ K, which is significantly higher than the apparent dust temperature of $T_d$ = 33 $\pm$ 2 K ($\beta$ = 1.9) derived under the common assumption of an optically thin far-infrared dust emission, but fully consistent with $T_d$ = 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 ∼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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Isabella Cortzen, Georgios E. Magdis, Francesco Valentino, Emanuele Daddi, Daizhong Liu, et al.. Deceptively cold dust in the massive starburst galaxy GN20 at z ∼ 4. Astronomy and Astrophysics - A&A, EDP Sciences, 2020, 634, pp.L14. ⟨10.1051/0004-6361/201937217⟩. ⟨cea-02496332⟩

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