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

TL;DR

This paper reports the discovery of the highest-redshift Lyman continuum emitter candidate at z=4.444, combining multi-wavelength data to analyze its properties, morphology, and LyC escape fraction, providing insights into galaxy reionization.

Contribution

It presents the first confirmed detection of a Lyman continuum emitter at z > 4 with detailed multi-instrument analysis and characterization of its physical properties and environment.

Findings

LyC emission detected at 3.7σ and 2.8-3.0σ significance in independent datasets.

Estimated LyC escape fractions around 33-38%.

Galaxy is compact, starbursting, with high star formation rate surface density.

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 the highest-redshift LCE to date with LyC detections confirmed in two independent data sets. The redshift is determined from the $Ly\alpha$ emission line detected in the VLT/MUSE spectrum. The LyC signal is detected independently in the Hubble Space Telescope (HST) F435W image and the VLT/MUSE spectrum at significances of $\simeq 3.7~\sigma$ and $\simeq 2.8 - 3.0~\sigma$, respectively. The centroid of the LyC emission is closely aligned with the rest-frame optical continuum traced by James Webb Space Telescope (JWST) imaging, with an offset of $\simeq 0''06$ (0.4 kpc in physical scale). Based on HST/ACS F435W photometry and MUSE spectroscopy, we infer LyC escape fractions of $f_{esc}(F435W) = 0.38_{-0.15}^{+0.25}$ and $f_{esc}(MUSE) = 0.33_{-0.13}^{+0.22}$. Using the combined JWST and MUSE data set, we characterize the physical properties and morphology of LCEz4-M1. The galaxy is compact and lies in the starburst regime, with a high star formation rate surface density of $\Sigma_{\rm SFR} \simeq 7~M_{\odot}~{\rm yr}^{-1}~{\rm kpc}^{-2}$, consistent with conditions that can drive strong feedback and outflows. The feedback may generate low-column-density pathways in the interstellar medium that facilitate LyC escape. While we find no clear evidence for an ongoing major merger, the presence of a faint companion ($\sim 0.''5$) detected in the F277W band suggests a potential minor interaction. This is also consistent with LCEz4-M1 residing in an overdense environment, where elevated interaction rates and dynamical perturbations are expected.