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 the highest-redshift LCE to date with LyC detections confirmed in two independent data sets. The redshift is determined from the $Lyα$ 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 $\simeq 3.7~σ$ and $\simeq 2.8 - 3.0~σ$, 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 $Σ_{\rm SFR} \simeq 7~M_{\odot}~{\rm yr}^{-1}~{\rm kpc}^{-2}$, 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.

Figures (3)

• Figure 1: The MUSE spectrum, SED, and multi-band cutouts of LCEz4-M1. (a) The full MUSE spectrum. No emission lines other than Ly$\alpha$ are detected. (b) The Ly$\alpha$ emission line in the MUSE spectrum. (c) The HST ACS/WFC F606W image, whose bandpass covers the Ly$\alpha$ line. (d) The Ly$\alpha$ narrowband image extracted from the MUSE data cube. (e) The SED and best-fit model from CIGALE. Blue points represent photometry from HST and ground-based telescopes, red points represent JWST photometry, open squares show the model-predicted fluxes, and downward arrows indicate upper limits from VLT/ISAAC and Spitzer IRAC1/IRAC2. (f) Multi-band cutouts from HST ACS/WFC F775W to JWST/NIRCam F480M that are used in the SED fitting. Red crosshairs mark the source position in all images.
• Figure 2: LyC emission of LCEz4-M1 detected in both HST and MUSE data. (a) HST ACS/WFC F435W cutout from the HLF dataset. Circular apertures are overplotted to indicate LCEz4-M1 (red) and nearby sources (white), with a radius of $0\farcs35$. The LyC emission is detected at $\mathrm{S/N} \sim 3.7$. (b) MUSE narrowband image constructed from the MUSE-HUDF DR2 cube covering the LyC region. The LyC emission is detected at $\sim 2.8-3.0 ~\sigma$. (c) JWST/NIRCam F277W cutout from the JADES dataset, probing the rest-frame $\sim 5000$ Å continuum and the [O iii] emission line. The green aperture of $0".15$ radius indicates the companion source of LCEz4-M1. (d) MUSE spectrum of the LyC region with the best-fit spectrum from pyPlatefitBacon2023. The gray shaded region shows the 1 $\sigma$ error.
• Figure 3: Morphological analysis of LCEz4-M1 using the JWST/NIRCam F200W data. The source is fitted both with a single-Sérsic model and with a two-component Sérsic model. The two rows show the fitting results for the single-component and two-component models, respectively. The reduced chi-squared values are similar for the two fits ($\chi^2_\nu \approx 0.5$).