Direct Lyman continuum and Lyman-alpha escape observed at redshift 4

TL;DR

This paper reports the discovery of a high-redshift galaxy at z=4 showing significant Lyman continuum leakage and Lyman-alpha emission, providing insights into ionising radiation escape mechanisms relevant for cosmic reionization.

Contribution

It presents the first detailed observation of a z=4 galaxy with copious LyC leakage and shared ionised cavity, advancing understanding of ionising photon escape at high redshift.

Findings

LyC escape fraction of ~60% observed in Ion3

Ion3 has a stellar mass of 1.5x10^9 Msun and SFR of 140 Msun/yr

Shared ionised cavity for LyC and Lya radiation at high redshift

Abstract

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% escaping), a remarkable multiple peaked Lya emission, and significant Lya radiation directly emerging at the resonance frequency. This is the highest redshift confirmed LyC emitter in which the ionising and Lya radiation possibly share a common ionised cavity (with N_HI<10^17.2 cm^-2). Ion3 is spatially resolved, it shows clear stellar winds signatures like the P-Cygni NV1240 profile, and has blue ultraviolet continuum (\beta = -2.5 +/- 0.25, F_\lambda~ \lambda^\beta) with weak low-ionisation interstellar metal lines. Deep VLT/HAWKI Ks and Spitzer/IRAC 3.6um and 4.5um imaging show a clear photometric signature of the Halpha line with equivalent width of 1000A rest-frame emerging over a flat continuum (Ks-4.5um ~ 0). From the SED fitting we derive a stellar mass of 1.5x10^9 Msun, SFR of 140 Msun/yr 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 reionising sources at z>6.5 with JWST.