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Dusty star-forming galaxies emit most of their light at far-IR to mmwavelengths as their star formation is highly obscured. Far-IR and mmobservations have revealed their dust, neutral and molecular gas properties.The sensitivity of JWST at rest-frame optical and near-infrared wavelengths nowallows the study of the stellar and ionized gas content. We investigate thespatially resolved distribution and kinematics of the ionized gas in GN20, adusty star forming galaxy at $z$=4.0548. We present deep MIRI/MRS integralfield spectroscopy of the near-infrared rest-frame emission of GN20. We detectspatially resolved \paa, out to a radius of 6 kpc, distributed in a clumpymorphology. The star formation rate derived from \paa\ (144 $\pm$ 9\msunperyear) is only 7.7 $\pm 0.5 $\% of the infrared star formation rate(1860 $\pm$ 90 \msunperyear). We attribute this to very high extinction (A$_V$= 17.2 $\pm$ 0.4 mag, or A$_{V,mixed}$ = 44 $\pm$ 3 mag), especially in thenucleus of GN20, where only faint \paa\ is detected, suggesting a deeply buriedstarburst. We identify four, spatially unresolved, clumps in the \paa\emission. Based on the double peaked \paa\ profile we find that each clumpconsist of at least two sub-clumps. We find mass upper limits consistent withthem being formed in a gravitationally unstable gaseous disk. The UV brightregion of GN20 does not have any detected \paa\ emission, suggesting an age ofmore than 10 Myrs for this region of the galaxy. From the rotation profile of\paa\ we conclude that the gas kinematics are rotationally dominated and the$v_{rot}/\sigma_{m} = 3.8 \pm 1.4$ is similar to low-redshift LIRGs. Wespeculate that the clumps seen in GN20 could contribute to building up theinner disk and bulge of GN20.

Clumpy star formation and an obscured nuclear starburst in the luminous dusty z=4 galaxy GN20 seen by MIRI/JWST