Galaxy Underdensities Host the Clearest IGM Ly$α$ Transmission and Indicate Anisotropic Reionization

TL;DR

This study uses JWST/NIRCam WFSS to map [O III] emitters around two highly Lyα-transparent quasar sightlines at $z \sim 5.7$, testing whether clearest IGM transmission resides in galaxy underdensities. The authors measure a 1D opacity–density relation and, crucially, a 2D cross-correlation revealing an off-axis, anisotropic transmission enhancement that challenges isotropic bubble growth in reionization models. Forward-modeling with FlexRT simulations shows general agreement on scatter and density–transmission trends but cannot fully reproduce the observed 2D anisotropy, implying directional escape and filamentary ionized structures shaped by the cosmic web. These results imply a complex, multi-pathway geometry for late-stage reionization and highlight the value of combining 2D Lyα transmission tomography with deep spectroscopic surveys to constrain the ionization topology. The findings have implications for interpreting Lyα forest transmission and the role of anisotropic feedback in reionization timing and structure.

Abstract

How galaxies drive reionization and what governs its geometry remain fundamental questions. We present JWST/NIRCam wide-field slitless spectroscopy (WFSS) observations toward two of the most Ly$α$-transmissive QSO sightlines near the end of reionization. We find that regions at $z \sim 5.7$ along both sightlines previously found to be low-density in Ly$α$ emitters are also underdense in [O III] emitters, with densities less than half the cosmic mean. Other transmissive regions, however, are found to coincide with average-density environments, indicating that multiple pathways may produce high IGM transmission. For the first time, we measure the two-dimensional cross-correlation between IGM transmission and galaxy positions, revealing evidence for anisotropic ionization geometry. Specifically, we detect enhanced transmission at transverse distances of $Δr \sim 0.8$ times the mean free path, consistent with ionizing photons escaping preferentially along large-scale structures that are aligned with, but offset from, the line of sight. This anisotropic escape may contribute to the observed patchiness of reionization and challenges the assumption of isotropic ionized bubble growth in current models.

Figures (12)

• Figure 1: Sky distribution of [O iii] emitters overlaid on JWST/NIRCam F335M mosaics for the J359$-$06 (left) and J1306$+$0356 (right) fields. Symbols indicate spectroscopic redshift bins: green stars mark galaxies at $z \approx 5.7$, corresponding to the most transmissive Ly$\alpha$ forest region as identified by the HSC/NB816 filter in christenson_relationship_2023; blue squares show galaxies at $z < 5.68$; yellow triangles denote the $5.77 < z < 5.82$ overdensity in J359$-$06, visible in the southeast quadrant and associated with strong metal absorption (C iv, Si iv) in the QSO spectrum; red circles and dark red crosses represent galaxies at $z > 5.85$, with crosses indicating sources excluded from the galaxy-IGM cross-correlation analysis (see text for details). The background QSO in each field is marked with a red plus. Notably, the regions near the most transmissive sightlines (green stars) exhibit a deficit of [O iii] emitters.
• Figure 2: Ly$\alpha$ forest transmission spectra of J359$-$06 (top) and J1306$+$0356 (bottom), shown as a function of redshift (bottom axis) and observed wavelength (top axis). The black curves indicate normalized flux, with the thicker portions marking the redshift ranges used for the IGM-galaxy cross-correlation analysis. Symbols mark the redshifts of [O iii] emitters and their transverse distance to the sightline, using the same color and shape coding as in Figure \ref{['fig:image']}. Green stars highlight sources near $z \approx 5.7$. The dark gray shaded regions correspond to the NB816 filter coverage as used in christenson_relationship_2023, coinciding with the most transmissive segment in each sightline. The lighter gray bands show $\pm 25\ h^{-1}$ cMpc regions centered on the NB816 peak redshift. Blue dashed lines indicate the locations of metal absorbers (C iv, Si iv) identified in the XQR-30 spectra.
• Figure 3: Relation between Ly$\alpha$ forest opacity and galaxy density at $z \sim 5.7$. The dashed red star marks our measurement from the J359 and J1306 fields based on [O iii] emitters (shown as a single point since both fields yield similar values). Black symbols show LAE-based measurements from christenson_constraints_2021. The black curve and shaded bands denote the mean relation and scatter from the "$\dot{N}_{\rm ion} \propto L_{\rm UV}$" FlexRT model of gangolli_correlation_2025.
• Figure 4: Stacked one-dimensional cross-correlation between normalized transmission and distance to the nearest galaxy, shown as $T/\langle T \rangle - 1$ versus $R / \lambda_{\mathrm{mfp}}$. The blue curve shows the mean prediction from the FlexRT simulation over $5.3 < z < 5.9$ for the intermediate-luminosity and limited-FOV mock galaxy selection. The shaded band indicates the 16--84th percentile range across mock realizations. The orange curve shows our observed measurement combining the J1306+0356 and J359--06 sightlines. Gray points with error bars are reproduced from kakiichi_jwst_2025.
• Figure 5: Two-dimensional transmission maps. Left: Observed map of normalized Ly$\alpha$ transmission, shown as $(T / \langle T \rangle) - 1$, as a function of transverse separation $\Delta r$ and line-of-sight separation $\Delta d$ from galaxies, both normalized by the mean free path ($\lambda_{\rm mfp}$). A prominent horizontal band of enhanced transmission is visible at $\Delta r \sim 0.8\,\lambda_{\rm mfp}$, suggesting anisotropic structure in the IGM. Right: Corresponding map from mock sightlines generated from the FlexRT simulation, constructed by stacking 10,000 bootstrap realizations in the redshift range $5.3 < z < 5.9$. The simulated enhancement appears more isotropic, with the strongest signal near the lower-right quadrant.