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

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.