We report the discovery of an extreme galaxy overdensity at z=5.4 in the
GOODS-S field using JWST/NIRCam imaging from JADES and JEMS alongside
JWST/NIRCam wide field slitless spectroscopy from FRESCO. We identified
potential members of the overdensity using HST+JWST photometry spanning
λ=0.4−5.0 μm. These data provide accurate and
well-constrained photometric redshifts down to m≈29−30mag.
We subsequently confirmed N=81 galaxies at 5.2<z<5.5 using JWST
slitless spectroscopy over λ=3.9−5.0 μm through a
targeted line search for Hα around the best-fit photometric
redshift. We verified that N=42 of these galaxies reside in the field while
N=39 galaxies reside in a density around ∼10 times that of a random
volume. Stellar populations for these galaxies were inferred from the
photometry and used to construct the star-forming main sequence, where
protocluster members appeared more massive and exhibited earlier star formation
(and thus older stellar populations) when compared to their field galaxy
counterparts. We estimate the total halo mass of this large-scale structure to
be 12.6≲log10​(Mhalo​/M⊙​)≲12.8 using an empirical stellar mass to halo mass relation, which is
likely an underestimate as a result of incompleteness. Our discovery
demonstrates the power of JWST at constraining dark matter halo assembly and
galaxy formation at very early cosmic times.Comment: Resubmitted to ApJ based on reviewer report; main text has 15 pages,
6 figures and 1 table; appendix has 1 page, 2 figure sets, and 2 table