2024A&A...691A..87M

J1316+2614 at z = 3.613 is the UV-brightest (M_UV = -24.7) and strongest Lyman continuum-emitting ($ f_{\mathrm{esc}}^{\mathrm{LyC}} \approx 90\% $) star-forming galaxy known; it also shows signatures of inflowing gas from its blue-dominated Lyα profile. We present high-resolution imaging with the Hubble Space Telescope (HST) and the Very Large Telescope (VLT) of the LyC, Lyα, rest-UV, and optical emission of J1316+2614. Detailed analysis of the LyC and UV light distributions reveals compact yet resolved profiles, with LyC and UV morphologies showing identical half-light radii of r_eff ≃ 220 pc. The continuum-subtracted Lyα emission, obtained with the HST ramp-filter FR551N, reveals an extended filamentary structure of ≃6.0 kpc oriented south to north with only residual flux within the stellar core, suggesting a Lyα 'hole'. Our spectral energy distribution analysis shows that J1316+2614 is characterised by a young (5.7 ± 1.0 Myr), nearly un-obscured stellar population with a high star-formation rate (SFR = 898 ± 181 M_⊙ yr^–1) and a stellar mass of $ M_{\star}^{\mathrm{young}} = (4.8 \pm 0.3) \times 10^{9}\,M_{\odot} $. Additionally, the spectral energy distribution analysis supports the absence of an underlying old stellar population ($ M_{\star}^{\mathrm{old}} \leq 2.8 \times 10^{9}\,M_{\odot} $, 3σ). J1316+2614 presents remarkably high SFR and stellar mass surface densities of log($ \Sigma \mathit{SFR} [M_{\odot}\,\mathrm{yr}^{-1}\,\mathrm{kpc}^{-2}]) = 3.47 \pm 0.11 $ and log($ \Sigma M_{\star} [M_{\odot}\,\mathrm{pc}^{-2}]) = 4.20 \pm 0.06 $, respectively, which are among the highest observed in star-forming galaxies and are more typically observed in local young massive star clusters and globular clusters. Our findings indicate that J1316+2614 is a powerful, young, and compact starburst that is leaking a significant amount of LyC photons due to a lack of gas and dust within the starburst. We explored the conditions for gas expulsion using a simple energetic balance and find that, given the strong binding force in J1316+2614, a high star-formation efficiency (ϵ_SF ≥ 0.7) is necessary to explain the removal of gas and its exposed nature. Our results thus suggest a close link between high ϵ_SF and high $ f_{\mathrm{esc}}^{\mathrm{LyC}} $. This high efficiency can also naturally explain the remarkably high SFR, UV luminosity, and efficient mass growth of J1316+2614, which acquired at least 62% of its mass in the last 6 Myr. J1316+2614 may exemplify an intense, feedback-free starburst with a high ϵ_SF, similar to those proposed for UV-bright galaxies at high redshifts.