JADES: Resolving the Stellar Component and Filamentary Overdense Environment of Hubble Space Telescope (HST)-dark Submillimeter Galaxy HDF850.1 at z = 5.18

Abstract HDF850.1 is the brightest submillimeter galaxy (SMG) in the Hubble Deep Field. It is known as a heavily dust-obscured star-forming galaxy embedded in an overdense environment at z = 5.18. With nine-band NIRCam images at 0.8–5.0 μ m obtained through the JWST Advanced Deep Extragalactic Survey, we detect and resolve the rest-frame UV–optical counterpart of HDF850.1, which splits into two components because of heavy dust obscuration in the center. The southern component leaks UV and H α photons, bringing the galaxy ∼100 times above the empirical relation between infrared excess and UV continuum slope (IRX– β UV ). The northern component is higher in dust attenuation and thus fainter in UV and H α surface brightness. We construct a spatially resolved dust-attenuation map from the NIRCam images, well matched with the dust continuum emission obtained through millimeter interferometry. The whole system hosts a stellar mass of 10 10.8±0.1 M ⊙ and star formation rate (SFR) of 10 2.8±0.2 M ⊙ yr −1 , placing the galaxy at the massive end of the star-forming main sequence at this epoch. We further confirm that HDF850.1 resides in a complex overdense environment at z = 5.17–5.30, which hosts another luminous SMG at z = 5.30 (GN10). The filamentary structures of the overdensity are characterized by 109 H α -emitting galaxies confirmed through NIRCam slitless spectroscopy at 3.9–5 μ m, of which only eight were known before the JWST observations. Given the existence of a similar galaxy overdensity in the GOODS-S field, our results suggest that 50% ± 20% of the cosmic star formation at z = 5.1–5.5 occur in protocluster environments.