2026ApJ...996...19B

We present high-resolution (0$\mathop{.}\limits^{^{\prime\prime} }$13-0$\mathop{.}\limits^{^{\prime\prime} }$23) NOEMA observations of the dust continuum emission at 1.1 mm (rest frame 220 μm) and JWST/NIRCam and MIRI imaging of the z = 4.055 starburst galaxy GN20. The sensitive NOEMA imaging at 1.6 kpc resolution reveals extended dust emission, ≈14 kpc in diameter (r_e ≈ 2.5 kpc, b/a = 0.5), which is centrally asymmetric and clumpy. The dust emission is as extended as the stellar emission and molecular gas traced by ¹²CO(2-1), with a common center, and is brightest in the strongly obscured nuclear part of the galaxy. Approximately one-third of the total dust emission emerges from the nucleus and the most prominent clump to the south, with (only) 60% from the central 3.5 × 1.5 kpc (0$\mathop{.}\limits^{^{\prime\prime} }$5-0$\mathop{.}\limits^{^{\prime\prime} }$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 ¹²CO and dust emission, we derive ${M}_{{\rm{mol}}}=2.{9}_{-0.3}^{+0.4}\times 1{0}^{11}$f M_⊙, with ${\alpha }_{{\rm{CO}}}=2.{8}_{-0.3}^{+0.5}$. We find the extended dust implies a lower global dust optical depth than previously reported but a high dust mass of ${M}_{{\rm{dust}}}=5.{7}_{-0.6}^{+0.8}\times 1{0}^{9}$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 and dust masses being similarly distributed throughout the starburst. The latter highlights the importance of accounting for radiative-transfer effects when comparing molecular gas and dust distributions from different tracers.