2025MNRAS.538L..16Q

We investigate the detectability of Lyman-$\alpha$ (Ly $\alpha$) emission from galaxies at the onset of cosmic reionization, aiming to understand the conditions necessary for detecting high-redshift sources like JADES-GS-z13-1-LA at $z=13$. By integrating galaxy formation models with detailed intergalactic medium (IGM) reionization simulations, we construct high-redshift galaxy catalogues to model intrinsic Ly $\alpha$ profiles and assess their transmission through the IGM. For a galaxy with $M_{\rm UV}\sim -18.5$ like JADES-GS-z13-1-LA, our fiducial model predicts a Ly $\alpha$ transmission of ${\sim }13$ per cent and there is a probability of observing Ly $\alpha$ emission with an equivalent width $\gt 40$ Å of up to 10 per cent. We also explore how variations in the UV ionizing escape fraction, dependent on host halo mass or specific star formation rate, impact Ly $\alpha$ detectability. Our findings reveal that reionization morphology significantly influences detection chances - models where reionization is driven by low-mass galaxies can boost the detection probability to as much as 12 per cent, while those driven by massive galaxies tend to reduce ionized regions around faint emitters, limiting their detectability. Our findings remain robust when further accounting for stochastic star formation with the detection probability still spanning 3 per cent to 12 per cent. This study underscores the interplay between reionization morphology and intrinsic galaxy properties in interpreting high-redshift Ly $\alpha$ observations.