Witnessing an extreme, highly efficient galaxy formation mode with resolved Lyman-<i>α</i> and Lyman-continuum emission

J1316+2614 at z = 3.613 is the UV-brightest ( M UV = −24.7) and strongest Lyman continuum-emitting ( f esc LyC ≈ 90%) star-forming galaxy known; it also shows signatures of inflowing gas from its blue-dominated Ly α profile. We present high-resolution imaging with the Hubble Space Telescope (HST) and the Very Large Telescope (VLT) of the LyC, Ly α , 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 Ly α emission, obtained with the HST ramp-filter FR 551 N , reveals an extended filamentary structure of ≃6.0 kpc oriented south to north with only residual flux within the stellar core, suggesting a Ly α ‘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 ⋆ young = (4.8 ± 0.3) × 10 9 M ⊙ . Additionally, the spectral energy distribution analysis supports the absence of an underlying old stellar population ( M ⋆ old ≤ 2.8 × 10 9 M ⊙ , 3 σ ). J1316+2614 presents remarkably high SFR and stellar mass surface densities of log(Σ SFR [ M ⊙ yr −1 kpc −2 ]) = 3.47 ± 0.11 and log(Σ M ⋆ [ M ⊙ pc −2 ]) = 4.20 ± 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 esc 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.