Probing Direct Contributions of Galaxies and AGN to Cosmic Reionization in a Quasar Field J0226+0302 with JWST NIRCam and NIRSpec

Xiangyu Jin, Jinyi Yang, Feige Wang, Koki Kakiichi, Xiaohui Fan, Enrico Garaldi, Jaclyn B. Champagne, George D. Becker, Yongda Zhu, Yunjing Wu, Marianne Vestergaard, Huanqing Chen, Valentina D'Odorico, Anna-Christina Eilers, Jiamu Huang, Hyunsung D. Jun, Mingyu Li, Maria Pudoka, Wei Leong Tee, Minghao Yue, Huanian Zhang, Siwei Zou

First listed 2026-06-16 | Last updated 2026-06-12

Abstract

We present JWST Cycle 2 NIRCam and NIRSpec observations in a quasar field J0226+0302 at z=6.5412 to probe the direct connections between the intergalactic medium (IGM), galaxies, and AGN during reionization. This field was previously observed by the JWST ASPIRE program and eight [OIII]-emitting galaxies were detected at 5.3<z<6.4 with a single NIRCam pointing. Using new NIRCam and NIRSpec observations, we identify 65 additional line-emitting galaxies at 5.3<z<6.4. The IGM-galaxy cross-correlation function shows a ~2 sigma excess IGM transmission at ~10-40 cMpc from galaxies when compared with the average IGM transmission, suggesting a significant contribution from regions traced by star-forming galaxies to the local ionizing background during reionization. The IGM-galaxy cross-correlation function is consistent with THESAN simulations with an IGM neutral fraction of 5%-7% and an average ionizing photon escape fraction f_esc of 6% from galaxies. Among 49 line-emitting galaxies observed by NIRSpec, we identify four AGN through detection of broad H-alpha emission lines with an AGN fraction of (8+/-4)%. By measuring the IGM effective optical depth around the AGN and the IGM-AGN cross-correlation function, we find that the IGM transmission is higher within 5 cMpc/h of the AGN than around the majority of [OIII] emitters. We interpret the excess IGM transmission as resulting from the local radiation enhancement by the AGN, and estimate f_esc of 50%-100% of the AGN from the IGM-AGN cross-correlation function. Future JWST NIRSpec observations in quasar fields will yield a more constraining IGM-AGN cross-correlation function, providing further insights into the roles of galaxies and AGN in reionization.

Short digest

This paper uses new JWST NIRCam and NIRSpec observations of the z=6.5412 quasar field J0226+0302 to connect spectroscopically confirmed galaxies and AGN directly to Lyα forest transmission in the surrounding IGM. By adding 65 new line emitters to the eight already known systems, the authors build a 5.3<z<6.4 sample whose IGM-galaxy cross-correlation shows a ~2σ transmission excess on 10-40 cMpc scales, consistent with THESAN models with an IGM neutral fraction of 5%-7% and a galaxy escape fraction around 6%. Among 49 NIRSpec-observed emitters they identify four broad-Hα AGN, and the higher transmission within 5 cMpc/h of these sources implies a much stronger local ionizing contribution, with inferred AGN f_esc of roughly 50%-100%. The paper therefore offers a rare direct comparison in one z~6.5 field, suggesting that galaxies help sustain the local reionizing background while JWST-selected AGN can create even more transmissive zones nearby.

Key figures to inspect

• Figure 1. Use this map to orient the combined ASPIRE and GO 3325 footprint, the NIRSpec/MSA coverage, and the redshift-position distribution of all confirmed z>5.3 sources relative to the quasar sightline. It establishes why J0226+0302 is a uniquely dense field for measuring transverse IGM transmission against a single background quasar.
• Figure 2. This figure provides the core AGN-identification evidence: the NIRSpec spectra and line decompositions isolate the broad Hα components that distinguish the four AGN from the narrow-line galaxy population. It matters because the later IGM-AGN transmission results depend on these sources being securely classified as broad-line AGN.
• Figure 5. This is the main observational result for the galaxy sample, showing the IGM-galaxy cross-correlation and the excess Lyα forest transmission relative to the mean IGM on roughly 10-40 cMpc scales. It is the clearest figure for the scale, significance, and bootstrap uncertainties behind the claim that regions traced by star-forming galaxies contribute to the local ionizing background.
• Figure 7. This simulation comparison translates the observed cross-correlation into physical constraints by matching the J0226 measurement to THESAN snapshots. It is the figure to cite for the paper’s inferred neutral fraction of about 5%-7% and the corresponding galaxy escape fraction near 6%.
• Figure 12. This is the synthesis figure for the AGN result, directly comparing the IGM-AGN cross-correlation around the four broad-line AGN with the galaxy cross-correlation. It shows why the authors argue for enhanced local transmission and much higher AGN escape fractions, making it central to the claim that AGN can have a stronger immediate radiative impact than the broader line-emitter population.