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 Ly $\alpha$ 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 $\alpha$ 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 $z \sim 6$ , its detection suggests that the universe is $\gtrsim$ 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 $\lt\! 1\%$ , $\approx 5\%$ and $\approx 25\%$ 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 Ly $\alpha$ damping wing absorption extending to $\sim$ $1\,300\,$ Å, and a possible CII * $\lambda1335$ emission line. Our findings strongly motivate dedicated follow-up observations of JADES-GS-z13-1-LA to determine whether it hosts an AGN.