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

Journal article

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) 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 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.

Authors: Cortzen, I; Magdis, GE; Valentino, F; Daddi, E; Liu, D

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: EDP Sciences
• Journal: Astronomy and Astrophysics
• Volume: 634
• Article number: L14
• Publication date: 2020-02-10
• Acceptance date: 2020-02-04
• DOI: 10.1051/0004-6361/201937217
• EISSN: 1432-0746
• ISSN: 0004-6361
• Language: English
• Keywords: FFR