EIGER VII. The evolving relationship between galaxies and the intergalactic medium in the final stages of reionization

Daichi Kashino, Simon J. Lilly, Jorryt Matthee, Ruari Mackenzie, Anna-Christina Eilers, Rongmon Bordoloi, Robert A. Simcoe, Rohan P. Naidu, Minghao Yue, Bin Liu

First listed 2025-06-03 | Last updated 2025-06-03

Abstract

We present a comprehensive analysis of the relationship between galaxies and the intergalactic medium (IGM) during the late stages of cosmic reionization, based on the complete JWST EIGER dataset. Using deep NIRCam $3.5\,\mathrm{μm}$ slitless spectroscopy, we construct a sample of 948 [\OIII]$\lambda5008$-emitting galaxies with $-21.4\lesssim M_\mathrm{UV}\lesssim -17.2$ spanning $5.33<z<6.97$ along six quasar sightlines. We correlate these galaxies with \Lya\ and \Lyb\ transmission measured from high-resolution quasar spectra across multiple redshift intervals. We find clear redshift evolution in the correlation between galaxy density and transmission: it is suppressed in overdense regions at $z<5.50$, while enhanced at $5.70<z<6.15$. The intermediate range exhibits a transitional behavior. Cross-correlation measurements further reveal excess absorption within $\sim 8$\,cMpc of galaxies at low redshifts, and enhanced transmission at intermediate scales ($\sim$5--20\,cMpc) at $z>5.70$. Statistical tests using mock catalogs with realistic galaxy clustering but no correlation with the transmission field confirm that the observed correlations are unlikely to arise by chance. The evolving signals can be explained by stronger absorption in overdense regions, combined with the competing influences of local radiation fields and the rising background radiation. While local radiation dominates ionization of the surrounding IGM at earlier times, the background becomes increasingly important, eventually surpassing the impact of nearby galaxies. These results support an inside-out progression of reionization, with ionized regions originating around clustered, star-forming galaxies and gradually extending into underdense regions.

Short digest

Using the full EIGER program, the authors assemble 948 [O III]λ5008 emitters (−21.4≲MUV≲−17.2) at 5.33<z<6.97 from deep NIRCam 3.5 μm slitless spectroscopy along six quasar sightlines and cross-correlate them with Lyα/Lyβ transmission in high-resolution quasar spectra. They find a strong redshift evolution: overdense regions at z<5.50 show suppressed transmission and excess absorption within ~8 cMpc, while at 5.70<z<6.15 the transmission is enhanced on intermediate (~5–20 cMpc) scales, with a transitional regime between. Mock catalogs without galaxy–IGM coupling show these signals are unlikely by chance. The results support inside-out reionization, where local radiation around clustered galaxies dominates early and is later overtaken by a rising UV background that sets the IGM ionization state.

Key figures to inspect

• Figure 1: Inspect the sky maps and redshift–impact-parameter panels per field (J0100, J1148, J0148) to see where [O III] emitters cluster relative to quasar Lyα transmission spikes/gaps and to visualize scales affected by the field-of-view ‘forbidden region.’
• Figure 2: Repeat the field-by-field check for J1030, J159, and J1120; note the Lyα position marker in J1120 to judge where Lyβ contamination impacts the apparent transmission, and compare field-to-field variance in emitter density versus forest structure.
• Figure 3: Use the [O III] flux and luminosity distributions to gauge dynamic range and field-to-field depth, which feed directly into the selection function underlying the galaxy–forest cross-correlations.
• Figure 4: Examine completeness maps versus redshift to identify spatial systematics (e.g., horizontal trails from bright sources) and assess how inhomogeneous completeness might bias small- versus intermediate-scale correlation signals.