Escape fractions from unattenuated Ly$α$ emitters around luminous $z>6$ quasars

Minghao Yue, Anna-Christina Eilers, Jorryt Matthee, Rohan P. Naidu, Rongmon Bordoloi, Frederick B. Davies, Joseph F. Hennawi, Daichi Kashino, Ruari Mackenzie, Robert A. Simcoe

First listed 2025-07-07 | Last updated 2025-09-22

Abstract

Ionized proximity zones around luminous quasars provide a unique laboratory to characterize the Ly$α$ emission lines from $z>6$ galaxies without significant attenuation from the intergalactic medium (IGM). However, Ly$α$ line measurements for galaxies within high-redshift quasars' proximity zones have been rare so far. Here we present deep spectroscopic observations obtained with the NIRSpec/MSA instrument on the James Webb Space Telescope (JWST) of galaxies in two $z>6$ quasar fields. We measure the Ly$α$ line fluxes for 50 galaxies at $6<z<7$ with UV absolute magnitude $M_\text{UV}<-19$ (median $M_\text{UV}=-19.97$), among which 15 are located near the luminous quasars, i.e. within $Δv<2500\rm\,km\,s^{-1}$. We find that galaxies near the quasars show significant flux bluewards of the systemic Ly$α$ wavelength, and have higher Ly$α$ equivalent width compared to galaxies at similar redshifts that are not located within the quasars' environment. Our result indicates little or no redshift evolution for the Ly$α$-emitter fraction from $z\sim6.4$ to $z\sim5$. Leveraging the low IGM opacity in the quasars' vicinity, we evaluate the Ly$α$ escape fraction ($f_\text{esc}^{\text{Ly}α}$) of high-redshift galaxies. Our analysis suggests that galaxies at $\langle z\rangle\approx6.4$ have an average $f_\text{esc}^{\text{Ly}α}=0.14\pm0.04$. This value is consistent with reionization models where the Lyman continuum escape fraction is low $(f_\text{esc}^\text{LyC}\lesssim0.1)$ for luminous galaxies, and where the most luminous galaxies have only a minor contribution to the total ionizing photon budget.

Short digest

JWST/NIRSpec-MSA spectroscopy in the J0100+2802 and J1148+5251 fields measures Lyα for 50 galaxies at 6<z<7 (median M_UV=−19.97), including 15 within Δv<2500 km s−1 of the quasars. Galaxies in the quasar proximity zones show clear blue-side Lyα flux and higher equivalent widths than similar-redshift field galaxies, implying little to no evolution in the LAE fraction from z≈6.4 to z≈5. Using the low IGM opacity near the quasars, the authors infer an average Lyα escape fraction of 0.14±0.04 at ⟨z⟩≈6.4. This supports reionization scenarios with low LyC escape in luminous galaxies (f_esc^LyC≲0.1), suggesting bright galaxies contribute modestly to the ionizing budget.

Key figures to inspect

• Figure 1: Map the galaxy sample around J0100+2802 and J1148+5251 to see which targets fall within the quasar-centered proximity regions and how the 28 vs. 22 targets populate redshift and projected distance.
• Figure 2: Compare stacked Lyα profiles; the near-quasar stack shows blueward flux relative to systemic (set by [O III]), directly illustrating reduced IGM opacity inside the proximity zones versus foreground/background stacks.
• Figure 3: Inspect the LAE fraction versus redshift and environment; the quasar-vicinity subset exceeds the foreground/background and literature control, supporting elevated Lyα transmission and the claimed minimal evolution relative to z≈5.
• Figure 4: Examine the stacked Lyα of C IV emitters outside the quasar zones; the observed blue-side flux argues these systems generate their own ionized bubbles, informing how hard radiation fields or local overdensities affect Lyα escape.