Constraining reionization-era Ly$α$ escape with JELS-MUSE: a highly complete H$α$-selected sample at $z\sim6.1$

Abstract

The Ly$α$ escape fraction, $f_{\rm{esc}}^{\rm{Ly}α}$, probes both the interstellar medium (ISM) conditions governing ionizing photon escape and the rising neutral fraction of the IGM through the Epoch of Reionization (EoR). Characterising the intrinsic, ISM-driven distribution of $f_{\rm{esc}}^{\rm{Ly}α}$ before IGM attenuation becomes dominant is essential to interpret the observed decline in Ly$α$ visibility through the EoR. We present $f_{\rm{esc}}^{\rm{Ly}α}$ measurements for a highly complete, H$α$-flux-limited sample of 24 star-forming galaxies at $z \approx 6.1$, drawn from the JWST Emission Line Survey (JELS) and observed in Ly$α$ with VLT/MUSE as part of the JELS-MUSE Large Area Survey. We detect Ly$α$ in 12 of 24 sources and a Ly$α$ emitter fraction of $X_{\rm{Ly}α} = 33 \pm 12$ per cent using the canonical EW(Ly$α$) $> 25 \, \mathring{\rm{A}}$ definition. Incorporating non-detections via reverse Kaplan-Meier survival analysis yields $\langle f_{\rm{esc}}^{\rm{Ly}α} \rangle = 0.07^{+0.04}_{-0.03}$, consistent with an independent stacked-flux estimate of $0.08^{+0.02}_{-0.02}$. Using reionization simulations matched to the area, depth, and redshift range of our survey, we find that all galaxies are expected to experience broadly similar IGM transmission, so we postulate that the large scatter in $f_{\rm{esc}}^{\rm{Ly}α}$ reflects genuine ISM-driven variance rather than differences in the surrounding IGM. Among the detections, higher $f_{\rm{esc}}^{\rm{Ly}α}$ galaxies tend to have lower nebular dust attenuation, bluer UV slopes, and lower stellar mass, consistent with feedback-regulated escape through localised, low-column-density ISM channels around star-forming regions. These results benchmark intrinsic Ly$α$ escape at the end of reionization, against which IGM suppression at $z > 7$ can be interpreted.