JADES: A Prominent Galaxy Overdensity Candidate within the First 500 Myr

TL;DR

This study reports a prominent overdensity candidate at $z \approx 10.5$ in the JWST JADES GOODS--S field, comprising 18 galaxies within a ~3 arcmin radius and a density contrast of $\delta_{\rm gal} = 3.5$ (≈4× the field). Using ForcePho for multi-band photometry and Prospector SED fitting, the authors derive sizes, stellar masses ($\sim 0.6$–$3\times10^8\ M_\odot$), SFRs ($\sim 5\ M_\odot\,\mathrm{yr^{-1}}$), and UV slopes, finding most systems compact ($r_{\rm half} \sim 200$ pc) and broadly consistent with high-$z$ scaling relations; about one-third show close companions within $1$ pkpc, indicating enhanced interactions. Two members show potential Balmer breaks, suggesting either evolved stellar populations or LRDs, which would imply surprisingly mature populations or bursty SF histories in the first 500 Myr. Photometric Ly$\alpha$-transmission hints at an ionized bubble centered on the overdensity with a radius of order a few cMpc, providing a rare window into the onset of cosmic reionization, though spectroscopic confirmation is essential to map the 3D structure and test the bubble scenario. Comparisons with THESAN indicate such a pronounced overdensity is uncommon in current simulations unless galaxies are brightened, highlighting the need for spectroscopic follow-up and refined modeling of early environment effects.

Abstract

We report a galaxy overdensity candidate at $z\approx 10.5$ in the JWST Advanced Deep Extragalactic Survey (JADES). This overdensity contains 18 galaxies with consistent photometric redshifts and robust F115W dropouts within 8 comoving Mpc in projection. The galaxy number density is four times higher than the field expectation, accounting for one-third of comparably bright galaxies and nearly 50% of the total star formation rate at $10<z_\mathrm{phot}<12$ in the GOODS-S field. Two compact members of the overdensity show potential Balmer breaks suggestive of evolved stellar populations or little red dots (LRDs). One-third of galaxies have close companions or substructures within 1 kpc at consistent photometric redshifts, implying more frequent interactions in an overdense environment. Most galaxies have stellar masses of 0.6-3$\times10^8$ $M_\odot$, half-light radii of $\sim$200 pc, and star formation rates of $\sim$5 $M_\odot \mathrm{yr^{-1}}$, with no significant deviation from typical high-redshift scaling relations. We find tentative evidence for a spatially varying Ly$α$ transmission inferred photometrically, consistent with an emerging ionized bubble. This overdensity provides a rare opportunity for probing the environmental impact on galaxy evolution and the onset of cosmic reionization within the first 500 Myr.

Figures (7)

• Figure 1: Spatial distribution of galaxy candidates at $10 < z_\mathrm{phot} < 12$ from the JADES photometric sample, color-coded by photometric redshift derived with the EAZY package. Large circles mark bright sources with F356W fluxes above 8 nJy (29.1 AB mag); small circles mark sources between 5 and 8 nJy. The bold black polygon outlines the JADES/NIRCam GOODS--S footprint in F115W. Hatched regions indicate shallow F115W coverage; all other areas within the footprint reach a point-source depth of 8 nJy at $>7\sigma$ significance Johnson2026. Contours show the estimated density field at the mean density and 1$\sigma$, 2$\sigma$, 3$\sigma$, and 4$\sigma$ above the mean density of bright sources, derived with a kernel density estimator following Helton2024overdense. The overdensity is on the west side of the field, with a peak galaxy number density exceeding four times the mean. Three less secure objects within the overdense region, shown as large circles with dashed outlines, are excluded from the density estimate and subsequent analysis and are discussed in Appendix \ref{['sec:dubious']}. North is up, and east is to the left.
• Figure 2: Composite view of the $z\approx10.5$ overdensity. The central panel shows the relative projected positions of member galaxies, color-coded by rest-frame UV absolute magnitude $M_{\rm UV}$. The surrounding panels present JWST/NIRCam cutouts, using the F115W, F277W, and F444W bands as blue, green, and red, respectively, with F115W as the dropout band. Each cutout has a size of $0.75^{\prime\prime}\times0.75^{\prime\prime}$; the scale bar indicates $0.2^{\prime\prime}$ (0.8 pkpc). The upper left of each cutout shows object labels ordered by declination, in correspondence with the central panel, and the lower right shows their NIRCam IDs. Objects with close companions that are also F115W dropouts are marked by circles. Objects D and K (IDs 462894 and 428363) each consist of two components at consistent photometric redshifts. The blue companion of object F (ID 463424) is at $z_\mathrm{phot}=1.86$ and not associated with the object.
• Figure 3: SED of objects with potential Balmer breaks. The upper two panels show NIRCam photometry with error bars (black dots), the best-fit galaxy spectra from Prospector (red curves), and synthetic photometry using the best-fit spectra (gray squares). The lower panel shows the transmission curves of NIRCam filters. The Balmer break feature is unlikely to be caused by emission lines, as H$\beta$ and [O3] fall outside the F444W band at $z>9$, while other lines within F444W are rarely strong enough to produce the $\sim 65\%$ flux excess.
• Figure 4: (a) UV continuum slope $\beta$ versus absolute UV magnitude $M_{\mathrm{UV}}$. (b) Half-light radius versus stellar mass, where sizes are measured from multiband Sérsic fitting using ForcePho. (c) Star formation rate averaged over the past 30 Myr versus stellar mass. Blue points denote galaxies in the overdensity, gray points show field galaxies at similar redshifts ($10 < z_{\mathrm{phot}} < 12$) and brightness (F356W $> 8$ nJy), and red points highlight objects in the overdensity with potential Balmer breaks. We assume that the Balmer breaks arise from stellar populations in the SED fitting, although they could alternatively be produced by LRDs.
• Figure 5: Spatial distribution of Ly$\alpha$ transmission in the overdensity and comparison with simulations. (a) THESAN-1 neutral-hydrogen map around a $z = 10.5$ overdensity, with contours marking regions where the median ionized fraction exceeds 90%. Galaxies with rest-frame $U$ band absolute magnitude brighter than $-17.5\,$mag are shown in yellow. (b) Observed galaxy positions color-coded by their inferred Ly$\alpha$ factor. The Ly$\alpha$ factor quantifies Ly$\alpha$ transmissions as the ratio of inferred Ly$\alpha$ flux to that produced by the galaxies according to nebular models. We estimate them from photometry using prospector while fixing the redshift of all galaxies to the median photometric redshift $z = 10.5$. The red cross marks the geometric center of the overdensity. (c) Ly$\alpha$ factor versus comoving radius for two assumed redshifts ($z = 10.5$ and 11.0), showing a robust declining trend despite redshift uncertainties.