Quantifying the Escape of Lyα at z ≈ 5–6: A Census of Lyα Escape Fraction with Hα-emitting Galaxies Spectroscopically Confirmed by JWST and VLT/MUSE

Abstract

The James Webb Space Telescope provides an unprecedented opportunity for unbiased surveys of H α -emitting galaxies at z > 4 with the NIRCam's wide-field slitless spectroscopy (WFSS). In this work, we present a census of Ly α escape fraction ( f esc,Ly α ) of 165 star-forming galaxies at z = 4.9–6.3, utilizing their H α emission directly measured from FRESCO NIRCam/WFSS data. We search for Ly α emission of each H α -emitting galaxy in the Very Large Telescope/MUSE data. The overall f esc,Ly α measured by stacking is 0.090 ± 0.006. We find that f esc,Ly α displays a strong dependence on the observed UV slope ( β obs ) and E ( B − V ), such that the bluest galaxies ( β obs ∼ −2.5) have the largest escape fractions ( f esc,Ly α ≈ 0.6), indicative of the crucial role of dust and gas in modulating the escape of Ly α photons. f esc,Ly α is less well related to other parameters, including the UV luminosity and stellar mass, and the variation in f esc,Ly α with them can be explained by their underlying coupling with E ( B − V ) or β obs . Our results suggest a tentative decline in f esc,Ly α at z ≳ 5, implying increasing intergalactic medium attenuation toward higher redshift. Furthermore, the dependence of f esc,Ly α on β obs is proportional to that of the ionizing photon escape fraction ( f esc,LyC ), indicating that the escape of Ly α and ionizing photon may be regulated by similar physical processes. With f esc,Ly α as a proxy to f esc,LyC , we infer that UV-faint ( M UV > −16) galaxies contribute >70% of the total ionizing emissivity at z = 5–6. If these relations hold during the epoch of reionization, UV-faint galaxies can contribute the majority of UV photon budget to reionize the Universe.