We present a study of intergalactic medium (IGM) transmission at 4.5<z<6.5 using high-signal-to-noise JWST/NIRSpec prism spectroscopy of 143 galaxies at 5<z<7 from the CAPERS and JADES surveys. By comparing the observed flux blueward of Lyα emission line to the prediction of spectral energy distribution modeling, we directly measure the IGM transmission along the individual galaxy sightlines. The average transmission measured from these galaxy sightlines is consistent with previous measurements based on luminous quasars. Current NIRSpec spectroscopy is sufficiently deep to probe IGM transmission on single sightlines. We find evidence for a highly ionized structure, \bubble, at z∼5.75−6 in the GOODS-S field based on the analysis of a high-S/N spectrum of one galaxy, GS-18846, at z=6.335. The IGM transmission of GS-z6IS is 0.17±0.02, an order of magnitude higher than the average of previous measurements at this redshift. This structure has a line-of-sight scale of ∼110 cMpc and spatially extends over at least 21×17 cMpc2. GS-z6IS is associated with a known large-scale galaxy overdensity at the same redshift, whose member galaxies show enhanced Lyα visibility and a broader Lyα equivalent width distribution compared to field galaxies at similar redshift. This result supports the interpretation that Lyα overdensity can trace bubbles of increased IGM transmission, although environmental effects on galaxy properties may also contribute. Our study demonstrates that high-S/N galaxy spectra offer a powerful new approach to tracing ionized structures during the epoch of reionization.
@article{arxiv.2605.02539,
title = {Using Ly$\alpha$ Transmitted Spectrum to Probe IGM Transmission and Identify Ionized Structures in Cosmic Reionization},
author = {Weida Hu and Casey Papovich and Lu Shen and Pablo Arrabal Haro and Bren E. Backhaus and Nikko J. Cleri and Mark Dickinson and James S. Dunlop and Steven L. Finkelstein and Mauro Giavalisco and Anton M. Koekemoer and Vasily Kokorev and Sara Mascia and Lorenzo Napolitano and Laura Pentericci and Aaron Smith and Anthony J. Taylor and Xin Wang and L. Y. Aaron Yung},
journal= {arXiv preprint arXiv:2605.02539},
year = {2026}
}