Direct Lyman continuum and Ly α escape observed at redshift 4

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 a emission, and significant Ly a 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 NHI < 1017.2cm-2). Ion3 is spatially resolved, it shows clear stellar winds signatures like the P-Cygni NVλ1240 profile, and has blue ultraviolet continuum (β = -2.5 ± 0.25, Fλ~ λβ) 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 Hα line with equivalent width of 1000Årest-frame emerging over a flat continuum (Ks-4.5μm ≃ 0). From the SED fitting, we derive a stellar mass of 1.5 × 109M⊙, SFR of 140 M⊙yr-1and 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.