An Ionized Superstructure at Cosmic Dawn Revealed by a Foundation Model for Astrophysical Research

Abstract: The spatial structure of cosmic reionization remains poorly constrained. Using a foundation model trained for James Webb Space Telescope datasets, we identify a luminous galaxy at redshift $z=7.567$ showing strong Ly$α$ transmission at $z=6.80\pm0.07$. The transmission extends over $210.5<sup>{+55.7}_{-53.5}$ comoving megaparsecs, indicating an ionized superstructure with hydrogen neutral fraction $$x_{\rm HI}=(2.07<sup>{+0.29}_{-0.30})\times10<sup>{-6}$$, in an epoch when the cosmic mean neutral fraction is $\sim0.4$. Cosmological simulations suggest a probability of $\approx10<sup>{-5}$ for a random sightline to reproduce the signal. We find no significant galaxy overdensity associated with the transmission region, in tension with canonical inside-out reionization models. These observations severely challenge existing reionization models in the standard $Λ$CDM Universe, and provide the first example of deep learning discovering a previously unknown astrophysical phenomenon.