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

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.