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

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 mu 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 alpha photons, bringing the galaxy similar to 100 times above the empirical relation between infrared excess and UV continuum slope (IRX-beta(UV)). The northern component is higher in dust attenuation and thus fainter in UV and H alpha 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(circle dot) and star formation rate (SFR) of 10(2.8 +/- 0.2)M(circle dot) 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 alpha-emitting galaxies confirmed through NIRCam slitless spectroscopy at 3.9-5 mu 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.