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$α$ 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 $z \sim 6$, 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$α$ 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.

Figures (6)

• Figure 1: Properties of the reionisation models used in this work. Left: the volume-weighted mean ionised fraction vs. redshift (reionisation history), with the redshift of JADES-GS-z13-1-LA indicated. The late start, early start, and very early start models have ionised fractions of $< 1\%$, $\approx 5\%$, and $\approx 25\%$ at $z = 13$, respectively. Middle: mean transmission of the Ly$\alpha$ forest at $4.8 < z < 6$ compared to measurements from Becker2013 and Bosman2021. Right: CMB electron scattering optical depth. The late start model is slightly more than $1\sigma$ below the Planck measurement, while the early start case is within $1\sigma$ of the fiducial measurement and the re-analysis by deBelsunce2021. The very early start case is in more than $4\sigma$ tension with the fiducial Planck result.
• Figure 2: Median Ly$\alpha$ transmission as a function of wavelength in each of our reionisation models. The colours and line styles are the same as in Figure \ref{['fig:compare_models']}, and the shaded regions denote $1\sigma$ sightline-to-sightline scatter. We see a substantial difference in IGM damping wing transmission between reionisation models, especially close to systemic Ly$\alpha$.
• Figure 3: Top: An example fit of the spectrum model for the early start model, obtained from the median of the highest-likelihood fits from each sightline's dynesty posterior distribution for the early start reionisation model. The blue histogram indicates the PRISM SED from Witstok2024, to which we fit the model in our analysis. The shaded regions indicate $1\sigma$ uncertainties for each spectral bin. The black line indicates the model, and the red dashed line indicates the model with an additional CII* emission line with $\text{EW}_\text{CII*}=15~\text{Å}$. Bottom: residuals from the fit. The shaded regions indicate the same $1\sigma$ uncertainties as above. The red dashed histogram indicates residuals when the potential CII* line is included in the model. Note that the residuals are consistent with continuum fluctuations except around $\lambda=1335~\text{Å}$. Including the CII* emission line smooths out the residuals and makes them consistent with continuum fluctuations.
• Figure 4: Velocity offset and intrinsic equivalent width joint posteriors for all three reionisation histories. In each plot, the upper panel shows the $\Delta v$ PDF, the lower-left panel shows the $0.5\sigma$, $1\sigma$, $1.5\sigma$, and $2\sigma$ contours for the $\Delta v$-$\text{EW}$ joint PDF, and the lower-right panel shows the $\text{EW}$ PDF. (a) shows the posterior for late start, which favours high $\Delta v$ and $\text{EW}$. (b) shows the posterior for late start when we use a uniform $\Delta v$ prior on the interval $[0,1000]$ rather than $[0,500]$. We include this to show that the late start model favours $\Delta v \approx 500~\text{km/s}$ if we allow $\Delta v$ to go out to $1000~\text{km/s}$. In subsequent calculations, we use the posterior shown in (a). Note that the $\Delta v$ axis is rescaled in this plot. (c) shows the posterior for early start, which peaks around $\text{EW}\approx400~\text{Å}$ and favours lower $\Delta v$. (d) shows the posterior for very early start, which peaks around $\text{EW}\approx100~\text{Å}$ and favours lower $\Delta v$. Note that the $\text{EW}$ axis is rescaled in this plot.
• Figure 5: Values of $P(\text{AGN}|D)$, including uncertainties, for varying values of $r_{\xi_\text{ion}}$ and $f_\text{AGN}$. Our fiducial value, for $r_{\xi_\text{ion}}=2.88$ and $f_\text{AGN}=0.20$, is shown in black. We compare this to probabilities obtained with a lower $r_{\xi_\text{ion}}$ of $2.00$ to show how $r_{\xi_\text{ion}}$ affects the estimate. We also compare this to probabilities obtained with priors at the upper and lower 1$\sigma$ limits from Scholtz2025. These priors, along with our fiducial prior, are shown on the plot as dotted grey lines.