A 210 Megaparsec Ionized Structure at $z=6.8$ Revealed by an Astrophysics Foundation Model

Abstract

Reionization is the last major transition in the Universe that transforms the intergalactic medium (IGM) from neutral to highly ionized, yet its spatial structure remains poorly constrained. Using an astrophysics foundation model trained for James Webb Space Telescope datasets, we identify a luminous galaxy at redshift $z=7.567$ showing significant Ly$\alpha$ transmission at $z=6.80\pm 0.07$. This transmission, named ``the First Ly$\alpha$ transmission in the Reionization Epoch at $z\sim 7$" (FL$\alpha$RE-z7), indicates a cosmological-scale ionized structure extending over $210.5_{-53.5}^{+55.7}$ comoving Megaparsecs at cosmic dawn. The hydrogen neutral fraction of the ionized structure is $x_{\mathrm{HI}}=(2.07_{-0.30}^{+0.29})\times 10^{-6}$, about $10^5$ times lower than the cosmic mean at similar redshifts $(x_{\mathrm{HI}}\approx 0.4)$. Cosmological simulations suggest a probability of $\approx 10^{-5}$ for a random sightline to reproduce the transmission. We find no significant galaxy overdensity associated with the transmission region, in tension with canonical inside-out reionization models. FL$\alpha$RE-z7 severely challenges existing reionization models in the standard $\Lambda$CDM Universe, indicating extreme spatial fluctuation in ionizing radiation or IGM temperature. Our result marks the first example of a foundation model discovering a previously unpredicted astrophysical phenomenon, demonstrating the power of deep learning methods for mining large astronomical datasets and finding unexpected new astrophysics.

arXiv commentUpdated Figures and Acknowledgments