Resolving the Dusty Star-forming Galaxy GN20 at z = 4.055 with NOEMA and JWST: A Similar Distribution of Stars, Gas, and Dust Despite Distinct Apparent Profiles

We present high-resolution (0 . ″ 13–0 . ″ 23) NOEMA observations of the dust continuum emission at 1.1 mm (rest frame 220 μ m) and JWST/NIRCam and MIRI imaging of the z = 4.055 starburst galaxy GN20. The sensitive NOEMA imaging at 1.6 kpc resolution reveals extended dust emission, ≈14 kpc in diameter ( r e ≈ 2.5 kpc, b / a = 0.5), which is centrally asymmetric and clumpy. The dust emission is as extended as the stellar emission and molecular gas traced by 12 CO(2–1), with a common center, and is brightest in the strongly obscured nuclear part of the galaxy. Approximately one-third of the total dust emission emerges from the nucleus and the most prominent clump to the south, with (only) 60% from the central 3.5 × 1.5 kpc (0 . ″ 5–0 . ″ 2), implying that the starburst is very extended. The combined JWST and NOEMA morphology suggests GN20 experienced a recent interaction or merger, likely invigorating the starburst. The radial surface brightness profiles of the molecular gas and near-IR stellar emission are similar, while, in contrast, the dust emission appears significantly more concentrated. Through self-consistent radiative-transfer modeling of the integrated and resolved 12 CO and dust emission, we derive M mol = 2 . 9 − 0.3 + 0.4 × 1 0 11 f M ⊙ , with α CO = 2 . 8 − 0.3 + 0.5 . We find the extended dust implies a lower global dust optical depth than previously reported but a high dust mass of M dust = 5 . 7 − 0.6 + 0.8 × 1 0 9 M ⊙ and gas-to-dust ratio of ≈50. Furthermore, we show that the distinct apparent radial profiles of the gas and dust can be explained purely by radiative-transfer effects (differences in the radial optical depths and temperatures), and that the observations are consistent with the gas