Unraveling the Lyman Continuum Emission of Ion3: Insights from HST multi-band imaging and X-Shooter spectroscopy

Abstract

We provide a comprehensive analysis of Ion3, the most distant LyC leaker at $z=3.999$, using multi-band HST photometry and X-Shooter spectroscopy. Deep HST F390W imaging probe uncontaminated LyC flux blueward $\sim$880Å, while the non-ionizing UV 1500Å/2800Å~flux is probed with the F814W/F140W band. High angular resolution allows us to properly mask low-$z$ interlopers and prevent contamination of measured LyC radiation. We confirm the detection of LyC flux at SNR $\sim$3.5 and estimate the escape fraction of ionizing photons to be in the range $f_{\mathrm{esc},\mathrm{rel}}$ = 0.06 -- 1, depending on the adopted IGM attenuation. Morphological analysis reveals a clumpy structure made of two main components, with effective radii of R${}_{\mathrm{eff}}$ $\sim$180 pc and R${}_{\mathrm{eff}}$ < 100 pc, and a total estimated de-lensed area in the rest-frame 1600Å~of 4.2~kpc${}^2$. We confirm the presence of faint ultraviolet spectral features HeII$\lambda$1640, CIII]$\lambda$1907,1909 and [NeIII]$\lambda$3968, with rest-frame EW(HeII) = (1.6$\pm$0.7)Å and EW(CIII]) = (6.5$\pm$3)Å. From [OII]$\lambda$$\lambda$3726,3729 and [CIII]$\lambda$1909/CIII]$\lambda$1906 we derive electron densities $n_{\mathrm{e}}^{[\mathrm{OII}]}$ = 2300$\pm$1900 cm${}^{-3}$ and $n_{\mathrm{e}}^{\mathrm{CIII}]}$ > 10${}^4$ cm${}^{-3}$, corresponding to an ISM pressure log(P/k) > 7.90. Furthermore, we derive an intrinsic SFR(H$\alpha$) $\approx$ 77 M${}_{\odot }$ yr${}^{-1}$ (corresponding to ${\Sigma }_{\mathrm{SFR}}=20$~M${}_{\odot }$~yr${}^{-1}$~kpc${}^{-2}$ for the entire galaxy) and sub-solar metallicity $12+\log (O/H)$ = 8.02$\pm$0.20 using the EW(CIII]) as a diagnostic. The detection of [NeIII]$\lambda$3968 line and [OII]$\lambda$$\lambda$3726,3729, provide an estimate of the ratio [OIII]$\lambda$5007/[OII]$\lambda$$\lambda$3727,29 of O32 > 50 and high ionization parameter log$U$ > $-$1.5 using empirical and theoretical correlations.

12 pages, 12 figures, Accepted to A&A

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