Unraveling the Lyman continuum emission of Ion3: Insights from HST multiband imaging and X-Shooter spectroscopy

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 kpc 2 . We confirm the presence of faint ultraviolet spectral features, including HeII λ 1640, CIII] λ 1907,1909, and [NeIII] λ 3968, with a rest-frame equivalent width EW(HeII) =(1.6 ± 0.7) Å and EW(CIII]) =(6.5 ± 3) Å. From the [OII] λλ 3726,3729 and [CIII] λ 1909/CIII] λ 1906 doublets, we derived electron densities of n e [OII] = 2300 ± 1900 cm −3 and n e [CIII] > 10 4 cm −3 , corresponding to an interstellar medium (ISM) pressure log(P/k) > 7.90. Furthermore, we derived an intrinsic SFR(H α ) ≈77 M ⊙ yr −1 (corresponding to Σ SFR = 20 M ⊙ yr −1 kpc −2 for the entire galaxy) and subsolar metallicity 12 + log(O/H) = 8.02 ± 0.20 using the EW(CIII]) as a diagnostic. The detection of [NeIII] λ 3968 line and [OII] λλ 3726,3729 offer an estimate of the ratio [OIII] λ 5007/[OII] λλ 3727,29 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.