Age dependence of Lyα escape fraction of Lyα emitters and their significant role in cosmic reionization

Abstract We study the Lyα escape fraction, $f_{\mathrm{esc}}^{\mathrm{Ly\alpha }}$of Lyα emitters (LAEs) identified by Subaru/HSC narrowband imaging at z = 2.2-6.6, using publicly available deep imaging data from HST and JWST. We perform SED fitting for 127 LAEs at 0.4 − 5.0 μm to estimate their physical properties robustly, and confirm that two distinct LAE populations exist: young LAEs (< 100 Myr) and old LAEs (> 100 Myr). Young LAEs are characterized by burst-like star formation activity and low dust content, significantly differing from Lyman-break galaxies (LBGs) at the same stellar mass, while old LAEs show similar star formation activity to LBGs, yet with lower dust content and more compact morphology in rest-UV/optical than LBGs. The $f_{\mathrm{esc}}^{\mathrm{Ly\alpha }}$of LAEs is anticorrelated with stellar mass, and this correlation is found to depend on the age of LAEs, such that old LAEs show a weaker anticorrelation than young LAEs, and tend to exhibit higher $f_{\mathrm{esc}}^{\mathrm{Ly\alpha }}$than young LAEs at a given stellar mass. This implies that Lyα photons escape more efficiently from old LAEs, possibly through the low-density channels of H i and dust created by outflows. The average $f_{\mathrm{esc}}^{\mathrm{Ly\alpha }}$of young LAEs remains nearly constant at ∼40% at z = 2.2 − 6.6, suggesting that the previously observed evolution of global $f_{\mathrm{esc}}^{\mathrm{Ly\alpha }}$of star-forming galaxies (SFGs) is due to the changes in the LAE fraction among the SFGs. Converting $f_{\mathrm{esc}}^{\mathrm{Ly\alpha }}$to Lyman continuum escape fraction using empirical relations, we demonstrate that LAEs alone can supply the ionizing photons necessary for reionization at z ∼ 6, causing rapid and late reionization.