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
μm obtained through the JWST Advanced Deep Extragalactic Survey (JADES), 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 Hα photons, bringing the galaxy
∼100 times above the empirical relation between infrared excess and UV
continuum slope (IRX-βUV​). The northern component is higher in
dust attenuation and thus fainter in UV and Hα 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
1011.0±0.1M⊙​ and star-formation rate of
103.0±0.2M⊙​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 Hα-emitting galaxies
confirmed through NIRCam slitless spectroscopy at 3.9-5 μm, 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.Comment: 44 pages, 16 figures, 2 tables. Resubmitted to ApJ after including
the first-round referee's comment