A long time ago in an LAE far, far away: a signpost of early reionisation or a nascent AGN at z = 13?

Abstract The JADES survey recently reported the discovery of JADES-GS-z13-1-LA at z = 13, the highest redshift Lyα emitter (LAE) ever observed. This observation suggests that either the intergalactic medium (IGM) surrounding JADES-GS-z13-1-LA is highly ionised, or the galaxy’s intrinsic Ly α emission properties are extreme. We use radiative transfer simulations of reionisation that capture the distribution of ionised gas in the z = 13 IGM to investigate the implications of JADES-GS-z13-1-LA for reionisation. We find that if JADES-GS-z13-1-LA is a typical star forming galaxy (SFG) with properties characteristic of LAEs at z ∼ 6, its detection suggests that the universe is ≳ 5% ionised by z = 13. We also investigate the possibility that the extreme properties of JADES-GS-z13-1-LA are driven by an AGN. Using a simple analysis based on the fact that AGN are expected to produce more ionising photons than SFGs, we estimate that the probability that JADES-GS-z13-1-LA hosts an AGN is 71%, 42%, and 15% if the IGM is < 1%, ≈ 5% and ≈ 25% ionised, respectively. We also highlight other features in the spectrum of JADES-GS-z13-1-LA that may be indicative of AGN activity, including strong Lyα damping wing absorption extending to ∼ 1300 Å, and a possible CII*λ1335 emission line. Our findings strongly motivate dedicated follow-up observations of JADES-GS-z13-1-LA to determine whether it hosts an AGN.