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

TL;DR

This study uses JWST spectroscopy to measure Lyα emission and escape fractions of galaxies near luminous $z>6$ quasars, revealing high Lyα flux and low escape fractions consistent with models of reionization and minimal contribution from luminous galaxies.

Contribution

First measurement of Lyα fluxes and escape fractions for galaxies near high-redshift quasars, providing new insights into galaxy ionizing photon output during reionization.

Findings

Galaxies near quasars show significant blueward Lyα flux and higher equivalent widths.

Estimated Lyα escape fraction for high-redshift galaxies is about 0.14.

Results support models with low LyC escape fractions and minor role of luminous galaxies in reionization.

Abstract

Ionized proximity zones around luminous quasars provide a unique laboratory to characterize the Ly$\alpha$ emission lines from $z>6$ galaxies without significant attenuation from the intergalactic medium (IGM). However, Ly$\alpha$ 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$\alpha$ 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 $\Delta v<2500\rm\,km\,s^{-1}$. We find that galaxies near the quasars show significant flux bluewards of the systemic Ly$\alpha$ wavelength, and have higher Ly$\alpha$ 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$\alpha$-emitter fraction from $z\sim6.4$ to $z\sim5$. Leveraging the low IGM opacity in the quasars' vicinity, we evaluate the Ly$\alpha$ escape fraction ($f_\text{esc}^{\text{Ly}\alpha}$) of high-redshift galaxies. Our analysis suggests that galaxies at $\langle z\rangle\approx6.4$ have an average $f_\text{esc}^{\text{Ly}\alpha}=0.14\pm0.04$. 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.