Using Ly$\alpha$ Transmitted Spectrum to Probe IGM Transmission and Identify Ionized Structures in Cosmic Reionization

TL;DR

This study uses high-quality JWST spectra of 143 galaxies at redshifts 5-7 to measure IGM transmission, revealing a large ionized bubble at z~5.75-6, and demonstrates a new method for probing cosmic reionization structures.

Contribution

It introduces a novel approach using galaxy spectra to directly measure IGM transmission and identify ionized structures during reionization, supported by JWST data.

Findings

Average IGM transmission aligns with quasar-based measurements.

Detected a large, highly ionized bubble at z~5.75-6.

Galaxy overdensity correlates with increased IGM transmission.

Abstract

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 Ly$\alpha$ 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 $z\sim 5.75-6$ 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 $0.17\pm0.02$, an order of magnitude higher than the average of previous measurements at this redshift. This structure has a line-of-sight scale of $\sim110$ cMpc and spatially extends over at least $21\times17$ cMpc$^2$. GS-z6IS is associated with a known large-scale galaxy overdensity at the same redshift, whose member galaxies show enhanced Ly$\alpha$ visibility and a broader Ly$\alpha$ equivalent width distribution compared to field galaxies at similar redshift. This result supports the interpretation that Ly$\alpha$ 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.