James Webb Space Telescope Feed Post

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

The NIRCam cutout images alongside the continuum-subtracted 2D and 1D grism spectra of JADES-GS+53.08831-27.84042 at z = 5.393. The upper left panel shows the 1farcs2 × 1farcs2 F444W–F277W–F150W RGB thumbnail. The upper right panel shows the extracted 2D spectrum around the Ha emission-line detection, indicated by the solid red line. The lower right panel instead shows the extracted 1D spectrum around the Ha emission-line detection alongside the best-fit Gaussian profile given by the solid red line. The JADES ID and confirmed spectroscopic redshift are given in the lower right panel for each galaxy. The complete figure set is available in the online journal. Abstract: We report the discovery of an extreme galaxy overdensity at z = 5.4 in the GOODS-S field using James Webb Space Telescope (JWST)/NIRCam imaging from JADES and JEMS alongside JWST/NIRCam wide-field slitless spectroscopy from FRESCO. We identified potential members of the overdensity using Hubble Space Telescope+JWST photometry spanning ? = 0.4–5.0 µm. These data provide accurate and well-constrained photometric redshifts down to m ˜ 29–30 mag. We subsequently confirmed N = 81 galaxies at 5.2 < z < 5.5 using JWST slitless spectroscopy over ? = 3.9–5.0 µm through a targeted line search for Ha 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\lesssim {\mathrm{log}}_{10}\left({M}_{\mathrm{halo}}/{M}_{\odot }\right)\lesssim 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.