JWST/MIRI unveils the stellar component of the GN20 dusty galaxy overdensity at z = 4.05

¹ Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir km 4, Torrejón de Ardoz, E-28850 Madrid, Spain
² Department of Astronomy, Stockholm University, Oscar Klein Centre, AlbaNova University Centre, 106 91 Stockholm, Sweden
³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁴ I.Physikalisches Institut der Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
⁵ Centro de Astrobiología (CAB), CSIC-INTA, Camino Viejo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁶ Telespazio UK for the European Space Agency, ESAC, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Spain
⁷ Cosmic Dawn Center, DTU Space, Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark
⁸ Cosmic Dawn Centre, Copenhagen, Denmark
⁹ Department of Physics and Astronomy, University College London, Gower Place, London WC1E 6BT, UK
¹⁰ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹¹ Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800 9700 AV Groningen, The Netherlands
¹² European Space Agency, Space Telescope Science Institute, Baltimore, Maryland, USA
¹³ DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 128 2200 Copenhagen, Denmark
¹⁴ School of Physics & Astronomy, Space Park Leicester, University of Leicester, 92 Corporation Road, Leicester LE4 5SP, UK
¹⁵ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
¹⁶ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, F-91191 Gif-sur-Yvette, France

^⋆ Corresponding author; acrespo@cab.inta-csic.es

Received: 26 February 2024
Accepted: 18 September 2024

Abstract

Dusty star-forming galaxies (DSFGs) at z > 2 have been commonly observed in overdense regions, where the merging processes and large halo masses induce rapid gas accretion, triggering star formation rates (SFRs) up to ∼1000 M_⊙ yr⁻¹. Despite the importance of these DSFGs for understanding star formation in the early Universe, their stellar distributions, traced by the near-infrared (near-IR) emission, had remained spatially unresolved until the arrival of the JWST. In this work, we present, for the first time, a spatially resolved morphological analysis of the rest-frame near-IR (∼1.1–3.5 μm) emission in DSFGs traced with the JWST/MIRI F560W, F770W, F1280W, and F1800W filters. In particular, we studied the mature stellar component for the three DSFGs and a Lyman-break galaxy (LBG) present in an overdensity at z = 4.05. Moreover, we used these rest-frame near-IR images along with ultraviolet (UV) and (sub)-mm ancillary photometric data to model their spectral energy distributions (SEDs) and extract their main physical properties (e.g. M_*, SFR, A_V). The sub-arcsec resolution images from the JWST have revealed that the light distributions in these galaxies present a wide range of morphologies, from disc-like to compact and clump-dominated structures. Two DSFGs and the LBG are classified as late-type galaxies (LTGs) according to non-parametric morphological indices, while the remaining DSFG is an early-type galaxy (ETG). These near-IR structures contrast with their ultraviolet emission, which is diffuse and, in GN20 and GN20.2b, off-centred by ∼4 kpc. This result suggests that star formation takes place across the entire galaxy, while the UV light traces only those regions where the otherwise high internal extinction decreases significantly. The SED fitting analysis yields large SFRs (∼300–2500 M_⊙ yr⁻¹), large stellar masses (M_* = (0.24–1.79) × 10¹¹ M_⊙), and high integrated extinction values (A_V = 0.8–1.5 mag) for our galaxies. In particular, we observe that GN20 dominates the total SFR with a value 2550 ± 150 M_⊙ yr⁻¹, while GN20.2b has the highest stellar mass (M_* = (2.2 ± 1.4) × 10¹¹ M_⊙). The two DSFGs classified as LTGs (GN20 and GN20.2a) have a high specific SFR (sSFR > 30 Gyr⁻¹), placing them above the star-forming main sequence (SFMS) at z ∼ 4 by ∼0.5 dex; whereas the ETG (i.e. GN20.2b) is compatible with the high-mass end of the main sequence. In comparison with other DSFGs in overdensities at z ∼ 2–7, we observe that our objects present similar SFRs, depletion times, and projected separations. Nevertheless, the sizes computed for GN20 and GN20.2a are up to two times larger than those of isolated galaxies observed in CEERS and ALMA-HUDF at similar redshifts. We interpret this difference in size as an effect of rapid growth induced by the dense environment.