2024ApJ...974...41H

We conduct a systematic search for high-redshift galaxy overdensities at 4.9 < z_spec < 8.9 in both the Great Observatories Origins Deep Survey (GOODS)-N and GOODS-S fields using James Webb Space Telescope/Near-Infrared Camera (JWST/NIRCam) imaging from the JWST Advanced Deep Extragalactic Survey and JWST Extragalactic Medium-band Survey in addition to JWST/NIRCam wide field slitless spectroscopy from the First Reionization Epoch Spectroscopic Complete Survey. High-redshift galaxy candidates are identified using Hubble Space Telescope + JWST photometry spanning λ = 0.4-5.0 μm. We confirmed the redshifts for roughly a third of these galaxies using JWST spectroscopy over λ = 3.9-5.0 μm through identification of either Hα or $\left[\mathrm{OIII}\right]\lambda 5008$ around the best-fit photometric redshift. The rest-ultraviolet magnitudes and continuum slopes of these galaxies were inferred from the photometry: the brightest and reddest objects appear in more dense environments and thus are surrounded by more galaxy neighbors than their fainter and bluer counterparts, suggesting accelerated galaxy evolution within overdense environments. We find 17 significant (δ_gal ≥ 3.04, N_gal ≥ 4) galaxy overdensities across both fields (seven in GOODS-N and 10 in GOODS-S), including the two highest redshift spectroscopically confirmed galaxy overdensities to date at $\left\langle {z}_{\mathrm{spec}}\right\rangle =7.954$ and $\left\langle {z}_{\mathrm{spec}}\right\rangle =8.222$ (representing densities around ∼6 and ∼12 times that of a random volume). We estimate the total halo mass of these large-scale structures to be $11.5\leqslant {\mathrm{log}}_{10}\left({M}_{\mathrm{halo}}/{M}_{\odot }\right)\leqslant 13.4$ using an empirical stellar mass-to-halo mass relation, which are likely underestimates as a result of incompleteness. These protocluster candidates are expected to evolve into massive galaxy clusters with ${\mathrm{log}}_{10}\left({M}_{\mathrm{halo}}/{M}_{\odot }\right)\gtrsim 14$ by z = 0.