2018MNRAS.476L..15V

We report on the serendipitous discovery of a z = 4.0, M₁₅₀₀ = -22.20 star-forming galaxy (Ion3) showing copious Lyman continuum (LyC) leakage (∼60 per cent escaping), a remarkable multiple peaked Ly α emission, and significant Ly α radiation directly emerging at the resonance frequency. This is the highest redshift confirmed LyC emitter in which the ionizing and Ly α radiation possibly share a common ionized channel (with N_H I_ < 10^17.2 cm^–2). Ion3 is spatially resolved, it shows clear stellar winds signatures like the P-Cygni N Vλ1240 profile, and has blue ultraviolet continuum (β = -2.5 ± 0.25, F_λ ∼ λ^β) with weak low-ionization interstellar metal lines. Deep VLT/HAWKI Ks and Spitzer/IRAC 3.6 and 4.5µm imaging show a clear photometric signature of the H α line with equivalent width of 1000 Å rest-frame emerging over a flat continuum (Ks - 4.5µm ≃ 0). From the SED fitting, we derive a stellar mass of 1.5 x 10⁹ M_☉, SFR of 140 M_☉ yr^–1 and age of ∼10 Myr, with a low dust extinction, E(B - V) <= 0.1, placing the source in the starburst region of the SFR-M* plane. Ion3 shows similar properties of another LyC emitter previously discovered (z = 3.21, Ion2, Vanzella et al. 2016). Ion3 (and Ion2) represents ideal high-redshift reference cases to guide the search for reionizing sources at z > 6.5 with JWST.