A long time ago in an LAE far, far away: a signpost of early reionization 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 $< 1\%$, $\approx 5\%$ 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 $\sim 1300$ angstroms, 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.