Clumpy star formation and an obscured nuclear starburst in the luminous dusty <i>z</i> = 4 galaxy GN20 seen by MIRI/JWST
Arjan Bik, Javier Álvarez-Márquez, L. Colina, Alejandro Crespo Gómez, Florian Peißker, Fabian Walter, Leindert Boogaard, G. Östlin, T. R. Greve, G. Wright, A. Alonso‐Herrero, K. I. Caputi, Luca Costantin, A. Eckart, Steven Gillman, J. Hjorth, Edoardo Iani, Iris Jermann, Á. Labiano, D. Langeroodi, Jens Melinder, Pablo G. Pérez‐González, J. P. Pye, Pierluigi Rinaldi, Tuomo Tikkanen, P. van der Werf, M. Güdel, Th. Henning, Pierre-Olivier Lagage, T. P. Ray, E. F. van Dishoeck
Abstract
Dusty star-forming galaxies emit most of their light at far-infrared to millimeter wavelengths as their star formation is highly obscured. Far-infrared and millimeter observations have revealed their dust, neutral and molecular gas properties. The sensitivity of JWST at rest-frame optical and near-infrared wavelengths now allows the study of the stellar and ionized gas content. We investigate the spatially resolved distribution and kinematics of the ionized gas in GN20, a dusty star-forming galaxy at z = 4.0548. We present deep MIRI/MRS integral field spectroscopy of the near-infrared rest-frame emission of GN20. We detect spatially resolved Pa α , out to a radius of 6 kpc, distributed in a clumpy morphology. The star formation rate derived from Pa α (144 ± 9 M ⊙ yr −1 ) is only 7.7 ± 0.5% of the infrared star formation rate (1860 ± 90 M ⊙ yr −1 ). We attribute this to very high extinction ( A V = 17.2 ± 0.4 mag, or A V , mixed = 44 ± 3 mag), especially in the nucleus of GN20, where only faint Pa α is detected, suggesting a deeply buried starburst. We identify four, spatially unresolved, clumps in the Pa α emission. Based on the double peaked Pa α profile, we find that each clump consists of at least two sub-clumps. We find mass upper limits consistent with them being formed in a gravitationally unstable gaseous disk. The ultraviolet bright region of GN20 does not have any detected Pa α emission, suggesting an age of more than 10 Myr for this region of the galaxy. From the rotation profile of Pa α , we conclude that the gas kinematics are rotationally dominated and the v rot / σ m = 3.8 ± 1.4 is similar to low-redshift luminous infrared galaxies. From the Pa α kinematics, we cannot distinguish between a rotational profile of a large disk and a late stage merger mimicking a disk. We speculate that GN20 is in the late stage of a major merger, where the clumps in a large gas-rich disk are created by the major merger, while the central starburst is driven by the merger event.