LCEz4-M1: A Lyman Continuum Emitter Candidate at z = 4.444 in the MUSE Hubble Ultra Deep Field

Abstract

High-redshift Lyman continuum emitters (LCEs) are crucial for understanding how galaxies ionize the neutral hydrogen in the epoch of reionization. However, detected LCEs at $z>4$ are quite rare. Here we report an LCE candidate at $z = 4.444$, dubbed LCEz4-M1, which is one of the highest-redshift LCE candidates currently known. The redshift is determined from the Ly$α$ emission line detected in the VLT/MUSE spectrum. The Lyman continuum (LyC) signal is detected independently in the \emph{Hubble Space Telescope} (HST) F435W image and the VLT/MUSE spectrum at significances of $\simeq3.7~σ$ and $\simeq2.8-3.0~σ$, respectively. The LyC centroid is spatially consistent with the JWST/NIRCam continuum within the astrometric uncertainty. Adopting the maximum IGM transmission, we infer conservative lower-limit escape fractions of $f_{\rm esc}({\rm F435W}) = 0.82^{+0.13}_{-0.17}$ and $f_{\rm esc}({\rm MUSE}) = 0.75^{+0.18}_{-0.28}$. Using the combined JWST and MUSE data set, we characterize the physical properties and morphology of LCEz4-M1. In our fiducial JWST-only SED fit, the galaxy is compact but has a moderate current galaxy-integrated star formation surface density, $Σ_{\rm SFR}=0.38~M_{\odot}\,{\rm yr^{-1}\,kpc^{-2}}$, suggesting that it is not an extreme compact starburst under this fiducial interpretation. While we find no clear evidence for an ongoing major merger for LCEz4-M1, the presence of a faint companion ($\sim 0''5$) detected in the F277W band suggests a potential minor interaction. We also find that LCEz4-M1 may lie in a locally overdense region, although the environmental interpretation remains tentative. Finally, the low ${\rm SFR}_{10\,{\rm Myr}}/{\rm SFR}_{100\,{\rm Myr}}$ ratio, low Ly$α$ EW, and relatively weak rest-frame optical emission lines of LCEz4-M1 may indicate a post-burst LyC-leaking phase.