Mapping dusty galaxy growth at <i>z</i> &gt; 5 with FRESCO: detection of Hα in submm galaxy HDF850.1 and the surrounding overdense structures
Thomas Herard-Demanche, R. J. Bouwens, Pascal A. Oesch, Rohan P. Naidu, Roberto Decarli, Erica J. Nelson, Gabriel Brammer, Andrea Weibel, Mengyuan Xiao, Mauro Stefanon, Fabian Walter, Jorryt Matthee, R. A. Meyer, Stijn Wuyts, Naveen A. Reddy, Lucie E. Rowland, Ivana van Leeuwen, Pablo Arrabal Haro, H. Dannerbauer, Alice E. Shapley, John Chisholm, Pieter van Dokkum, Ivo Labbé, G. D. Illingworth, D. Schaerer, Irene Shivaei
Abstract
ABSTRACT We report the detection of a 13$\sigma$ H$\alpha$ emission line from HDF850.1 at $z=5.188\pm 0.001$ using the FRESCO (First Reionization Era Spectroscopically Complete Observations) NIRCam F444W grism observations. Detection of H$\alpha$ 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 F444W 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$\alpha$ emission appears to arise entirely from the less-obscured southern component and shows a similar $\Delta$v$\sim$ + 130 ${\rm{km\,s}}^{-1}$ velocity offset to that seen for [C ii] relative to the source systemic redshift. Leveraging H$\alpha$-derived redshifts from FRESCO observations, we find that HDF850.1 is forming in one of the richest environments identified to date at $z\gt 5$, with 100 $z=5.17$–5.20 galaxies distributed across 13 smaller structures and a $\sim$(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 $\gt 10^{14}$${\rm M}_{\odot }$ cluster by $z\sim 0$. Comparing galaxy properties forming within this overdensity with those outside, we find the masses, SFRs, and $UV$ luminosities inside the overdensity to be clearly higher. The prominence of H$\alpha$ line emission from HDF850.1 and other known highly obscured $z\gt 5$ galaxies illustrates the potential of NIRCam-grism programs to map both the early build-up of IR-luminous galaxies and overdense structures.