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

¹ Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, I-20133 Milano, Italy
² INAF – OAS, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Gobetti 93/3, I-40129 Bologna, Italy
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
⁵ INAF – IASF Milano, Via A. Corti 12, I-20133 Milano, Italy
⁶ Department of Astronomy, University of Massachusetts Amherst, Amherst, MA 01002, USA
⁷ INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00078 Monte Porzio Catone, Italy
⁸ Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Trieste, Via Tiepolo 11, Trieste, Italy
⁹ IFPU – Institute for Fundamental Physics of the Universe, Via Beirut 2, 34014 Trieste, Italy
¹⁰ Istituto Nazionale di Astrofisica, Vicolo dell’Osservatorio 5, 35137 Padova, Italy
¹¹ Università di Salerno, Dipartimento di Fisica “E.R. Caianiello”, Via Giovanni Paolo II, 132, I-84084 Fisciano (SA), Italy
¹² INAF – Osservatorio Astronomico di Capodimonte, Via Moiariello 16, I-80131 Napoli, Italy
¹³ INFN – Gruppo Collegato di Salerno – Sezione di Napoli, Dipartimento di Fisica “E.R. Caianiello”, Università di Salerno, via Giovanni Paolo II, 132, I-84084 Fisciano (SA), Italy
¹⁴ Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Ferrara, via Saragat 1, I-44122 Ferrara, Italy
¹⁵ Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800 9700AV Groningen, The Netherlands
¹⁶ Cosmic Dawn Center (DAWN), Copenhagen, Denmark

^⋆ Corresponding author.

Received: 22 August 2024
Accepted: 21 April 2025

Abstract

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_e^[OII] = 2300 ± 1900 cm⁻³ and n_e^[CIII] > 10⁴ 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 + log(O/H) = 8.02 ± 0.20 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.