JWST ASPIRE: How Did Galaxies Complete Reionization? Evidence for Excess IGM Transmission around ${\rm [O\,{\scriptstyle III}]}$ Emitters during Reionization

Koki Kakiichi, Xiangyu Jin, Feige Wang, Romain A. Meyer, Enrico Garaldi, Sarah E. I. Bosman, Frederick B. Davies, Xiaohui Fan, Maxime Trebitsch, Jinyi Yang, Eduardo Bañados, Jaclyn B. Champagne, Anna-Christina Eilers, Joseph F. Hennawi, Fengwu Sun, Yunjing Wu, Siwei Zou, Rahul Kannan, Aaron Smith, George D. Becker, Valentina D'Odorico, Thomas Connor, Weizhe Liu, Klaudia Protušová, Fabian Walter, Huanian Zhang

First listed 2025-03-10 | Last updated 2025-03-10

Abstract

The spatial correlation between galaxies and the Ly$α$ forest provides insights into how galaxies reionized the Universe. Here, we present initial results on the spatial cross-correlation between [OIII] emitters and Ly$α$ forest at 5.4<z<6.5 from the JWST ASPIRE NIRCam/F356W Grism Spectroscopic Survey in z>6.5 QSO fields. Using data from five QSO fields, we find $2σ$ evidence for excess Ly$α$ forest transmission at ~20-40 cMpc around [OIII] emitters at z=5.86, indicating that [OIII] emitters reside within a highly ionized IGM. At smaller scales, the Ly$α$ forest is preferentially absorbed, suggesting gas overdensities around [OIII] emitters. Comparing with models including THESAN simulations, we interpret the observed cross-correlation as evidence for significant large-scale fluctuations of the IGM and the late end of reionization at z<6, characterized by ionized bubbles over 50 cMpc around [OIII] emitters. The required UV background necessitates an unseen population of faint galaxies around the [OIII] emitters. Furthermore, we find that the number of observed [OIII] emitters near individual transmission spikes is insufficient to sustain reionization in their surroundings, even assuming all [OIII] emitters harbour AGN with 100 % LyC escape fractions. Despite broad agreement, a careful analysis of ASPIRE and THESAN, using the observed host halo mass from the clustering of [OIII] emitters, suggests that the simulations underpredict the observed excess IGM transmission around [OIII] emitters, challenging our model of reionization. Potential solutions include larger ionized bubbles at z<6, more enhanced large-scale UV background or temperature fluctuations of the IGM, and possibly a patchy early onset of reionization at z>10. Current observational errors are dominated by cosmic variance, meaning future analyses of more QSO fields from JWST will improve the results.

Short digest

ASPIRE cross-correlates [O III] emitters from NIRCam/F356W grism with Lyα-forest pixels in five z>6.5 QSO fields (5.4<z<6.5). It finds 2σ excess Lyα transmission at ~20–40 cMpc around [O III] emitters (z=5.86) with small-scale absorption, implying highly ionized bubbles around galaxies embedded in overdense environments. Comparing to THESAN, the signal points to >50 cMpc ionized regions and a stronger UV background supplied by unseen faint galaxies; the observed [O III] sources near individual transmission spikes cannot by themselves maintain the ionization even if all hosted AGN with 100% LyC escape. Simulations underpredict the excess transmission, hinting at larger bubbles or stronger UV/temperature fluctuations, with current uncertainties dominated by cosmic variance.

Key figures to inspect

• Figure 1 — Redshift distribution of [O III] emitters across the five QSO fields: verify how many lie within each sightline’s Lyα-forest window and where the cross-correlation has most leverage.
• Figure 2 — [O III] vs. UV luminosity: check whether the sample follows the Matthee (2023) trend and whether extreme [O III] strengths bias the emitters toward harder ionizing spectra relevant for the UVB.
• Figure 3 — Field-by-field Lyα transmission with emitter redshifts: visually assess the ~20–40 cMpc excess-transmission scales and the small-scale absorption near emitters; note proximity-zone masking and per-field variance.
• Figure 4 — Zoom on the J0224-4711 transmission spike: inspect the local overdensity of [O III] emitters relative to the spike and the line-of-sight/comoving separations that motivate the inference of large ionized regions.