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

¹ Department of Astronomy, University of Geneva, 51 Chemin Pegasi, 1290 Versoix, Switzerland
² CNRS, IRAP, 14 Avenue E. Belin, 31400 Toulouse, France
³ INAF – OAS, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Gobetti 93/3, I-40129 Bologna, Italy
⁴ Department of Astronomy, University of Texas at Austin, 2515 Speedway, Austin, Texas 78712, USA
⁵ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁶ Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir km 4, Torrejón de Ardoz, E-28850 Madrid, Spain

^⋆ Corresponding author; rui.marquescoelhochaves@unige.ch

Received: 26 July 2024
Accepted: 6 September 2024

Abstract

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 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_⋆^young = (4.8 ± 0.3) × 10⁹ M_⊙. Additionally, the spectral energy distribution analysis supports the absence of an underlying old stellar population (M_⋆^old ≤ 2.8 × 10⁹ M_⊙, 3σ). 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(ΣM_⋆[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.