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We report the discovery of an extreme galaxy overdensity at $z = 5.4$ in theGOODS-S field using JWST/NIRCam imaging from JADES and JEMS alongsideJWST/NIRCam wide field slitless spectroscopy from FRESCO. We identifiedpotential members of the overdensity using HST+JWST photometry spanning$\lambda = 0.4-5.0\ \mu\mathrm{m}$. These data provide accurate andwell-constrained photometric redshifts down to $m \approx 29-30\,\mathrm{mag}$.We subsequently confirmed $N = 81$ galaxies at $5.2 < z < 5.5$ using JWSTslitless spectroscopy over $\lambda = 3.9-5.0\ \mu\mathrm{m}$ through atargeted line search for $\mathrm{H} \alpha$ around the best-fit photometricredshift. We verified that $N = 42$ of these galaxies reside in the field while$N = 39$ galaxies reside in a density around $\sim 10$ times that of a randomvolume. Stellar populations for these galaxies were inferred from thephotometry and used to construct the star-forming main sequence, whereprotocluster members appeared more massive and exhibited earlier star formation(and thus older stellar populations) when compared to their field galaxycounterparts. We estimate the total halo mass of this large-scale structure tobe $12.6 \lesssim \mathrm{log}_{10} \left( M_{\mathrm{halo}}/M_{\odot} \right)\lesssim 12.8$ using an empirical stellar mass to halo mass relation, which islikely an underestimate as a result of incompleteness. Our discoverydemonstrates the power of JWST at constraining dark matter halo assembly andgalaxy formation at very early cosmic times.

The JWST Advanced Deep Extragalactic Survey: Discovery of an Extreme Galaxy Overdensity at $z = 5.4$ with JWST/NIRCam in GOODS-S