Quantifying the escape of Ly$\alpha$ at $z\approx 5-6$: a census of Ly$\alpha$ escape fraction with H$\alpha$ emitting galaxies spectroscopically confirmed by JWST and VLT/MUSE

Abstract

JWST provides an unprecedented opportunity for unbiased surveys of H$\alpha$-emitting galaxies at $z>4$ with the NIRCam wide-field slitless spectroscopy (WFSS). In this work, we present a census of Ly$\alpha$ escape fraction ($f_{esc,Ly\alpha }$) of 165 star-forming galaxies at $z=4.9-6.3$ using their H$\alpha$ emission directly measured from FRESCO NIRCam/WFSS data. We search for Ly$\alpha$ emission of each H$\alpha$-emitting galaxy in VLT/MUSE data. The overall $f_{esc,Ly\alpha }$ measured by stacking is $f_{esc,Ly\alpha }$ is $0.090\pm 0.006$. We find that $f_{esc,Ly\alpha }$ displays a strong dependence on the observed UV slope (${\beta }_{\mathrm{obs}}$) and E(B-V), such that the bluest galaxies (${\beta }_{\mathrm{obs}}\sim -2.5$) have the largest escape fractions ($f_{\mathrm{esc},\mathrm{Ly}\alpha }\approx 0.6$), indicative of the crucial role of dust and gas in modulating the escape of Ly$\alpha$ photons. $f_{esc,Ly\alpha }$ is less well related to other parameters, including the UV luminosity and stellar mass, and the variation in $f_{esc,Ly\alpha }$ with them can be explained by their underlying coupling with E(B-V) or ${\beta }_{\mathrm{obs}}$. Our results suggest a tentative decline in $f_{esc,Ly\alpha }$ at $z\gtrsim 5$, implying increasing intergalactic medium attenuation towards higher redshift. Furthermore, the dependence of $f_{esc,Ly\alpha }$ on ${\beta }_{\mathrm{obs}}$ is proportional to that of the ionizing photon escape fraction ($f_{\mathrm{esc},\mathrm{LyC}}$), indicating the escape of Ly$\alpha$ and ionizing photon may be regulated by similar physical processes. With $f_{esc,Ly\alpha }$ as a proxy to $f_{\mathrm{esc},\mathrm{LyC}}$, we infer that UV-faint ($M_{\mathrm{UV}}>-16$) 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.

17 pages, 12 figures; Accepted by the ApJS