Mapping dusty galaxy growth at z > 5 with FRESCO: detection of Hα in submm galaxy HDF850.1 and the surrounding overdense structures

We report the detection of a 13 σ H α emission line from HDF850.1 at z = 5 . 188 ± 0 . 001 using the FRESCO (First Reionization Era Spectroscopically Complete Observations) NIRCam F 444 W grism observations. Detection of H α in HDF850.1 is noteworthy, given its high far-infrared (IR) luminosity, substantial dust obscuration, and the historical challenges in deriving its redshift. HDF850.1 shows a clear detection in the F 444 W imaging data, distributed between a northern and southern component, mirroring that seen in [C II ] from the Plateau de Bure Interferometer. Modelling the spectral energy distribution of each component separately, we find that the northern component has a higher mass, star formation rate (SFR), and dust extinction than the southern component. The observed H α emission appears to arise entirely from the less-obscured southern component and shows a similar v ∼+ 130 km s −1 velocity offset to that seen for [C II ] relative to the source systemic redshift. Leveraging H α-derived redshifts from FRESCO observations, we find that HDF850.1 is forming in one of the richest environments identified to date at z > 5, with 100 z = 5 . 17–5.20 galaxies distributed across 13 smaller structures and a ∼(15 cMpc) 3 volume. Based on the evolution of analogous structures in cosmological simulations, the z = 5 . 17–5.20 structures seem likely to collapse into a single > 10 14 M cluster by z ∼ 0. Comparing galaxy properties forming within this o v erdensity with those outside, we find the masses, SFRs, and U V luminosities inside the o v erdensity to be clearly higher. The prominence of H α line emission from HDF850.1 and other known highly obscured z > 5 galaxies illustrates the potential of NIRCam-grism programs to map both the early build-up of IR-luminous galaxies and o v erdense structures.