MXDFz4.4: A LyC Emitter 250 Myr after the Epoch of Reionization and a First Test of Lyα Morphology as a Tracer of LyC Escape at High Redshift
July 2026 • 2026ApJ..1005...34G
Abstract • Assessing the contribution of ionizing sources to cosmic reionization is a central goal of extragalactic astrophysics. Understanding and quantifying ionizing escape remains challenging near the Epoch of Reionization. We present the highest-redshift Lyman continuum (LyC) emitter detected to date, MXDFz4.4, at z = 4.442 in the MUSE eXtremely Deep Field, observed only ∼0.25 Gyr after the end of reionization. A high-confidence Lyα line confirms the redshift. LyC flux is detected at 5.3σ in the F435W filter with a flux of 4.2 ± 0.8 nJy, corresponding to a flux measurement at 5.2σ. After correcting for the intrinsic production of LyC photons and the intergalactic medium opacity at z = 4.44, we derive high escape fractions, fesc, ranging from 50% to 100%. We apply established low-redshift tracers of LyC escape and, for the first time at this redshift, promising Lyα morphological tracers such as the halo fraction (HF). Spectral energy distribution fitting indicates the presence of a recent burst of star formation; we explore its impact on the production and escape of ionizing photons. Lyα-based tracers of fesc reveal a complex scenario in which the recent burst strongly influences LyC production and escape, combined with a more evolved stellar population. This interpretation is supported by UV diagnostics, including ΣSFR and sSFR. Our results provide cautious support for the Lyα HF as a LyC escape tracer at high redshift. Considering the burst-driven enhancement in ionizing photon production and escape, we conclude that stochastic star formation in the early Universe likely plays a significant role in the contribution of galaxies to cosmic reionization.
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