2025A&A...698A.203M

We provide a comprehensive analysis of Ion3, the most distant LyC leaker at z = 3.999, using multiband HST photometry (F390W, F814W, and F140W) and reevaluated X-Shooter spectroscopy. Deep HST F390W imaging enables us to probe uncontaminated LyC flux blueward of ∼880 Å. In this work, the nonionizing UV 1500 Å/2800 Å flux was probed with the F814W/F140W band. High angular resolution allowed us to properly mask low-z interlopers and prevent contamination of measured LyC radiation. We confirm the detection of LyC flux at a signal-to-noise ratio of S/N ∼ 3.5 and estimate the escape fraction of ionizing photons to be in the range f_esc, rel_ = 0.06-1, depending on the adopted IGM attenuation. A morphological analysis of Ion3 reveals a clumpy structure made up of two main components, Ion3_A and Ion3_B, with effective radii of R_eff ∼ 180 pc and R_eff < 100 pc, respectively, along with a total estimated delensed area in the rest-frame 1600 Å of 4.2 kpc². We confirm the presence of faint ultraviolet spectral features, including HeIIλ1640, CIII]λ1907,1909, and [NeIII]λ3968, with a rest-frame equivalent width EW(HeII) =(1.6 ± 0.7) Å and EW(CIII]) =(6.5 ± 3) Å. From the [OII]λλ3726,3729 and [CIII]λ1909/CIII]λ1906 doublets, we derived electron densities of $ n_{\mathrm{e}}^{\mathrm{[OII]}} = 2300\pm1900 $ cm^–3 and $ n_{\mathrm{e}}^{\mathrm{CIII]}} > 10^{4} $ cm^–3, corresponding to an interstellar medium (ISM) pressure log(P/k) > 7.90. Furthermore, we derived an intrinsic SFR(Hα) ≈77 M_⊙ yr^–1 (corresponding to Σ_SFR = 20 M_⊙ yr^–1 kpc^–2 for the entire galaxy) and subsolar metallicity $ 12+\rm log(O/H) = 8.02\pm0.20 $ using the EW(CIII]) as a diagnostic. The detection of [NeIII]λ3968 line and [OII]λλ3726,3729 offer an estimate of the ratio [OIII]λ5007/[OII]λλ3727,29 of O32 > 50 and a high ionization parameter, log U > -1.5, using empirical and theoretical correlations. These measurements imply remarkably high ionization levels and density conditions produced by an ongoing bursty star formation, observed during an ionizing, optically thin phase of the ISM along the line of sight.