JADES: Primeval Lyman-alpha emitting galaxies reveal early sites of reionisation out to redshift z ~ 9

Given the sensitivity of the resonant $\mathrm{Lyman}-\alpha$ ($\mathrm{Ly}\alpha$ ) transition to absorption by neutral hydrogen, observations of $\mathrm{Ly}\alpha$ emitting galaxies (LAEs) have been widely used to probe the ionising capabilities of reionisation-era galaxies and their impact on the intergalactic medium (IGM). However, prior to JWST our understanding of the contribution of fainter sources and of ionised “bubbles” at earlier stages of reionisation remained uncertain. Here, we present the characterisation of three exceptionally distant LAEs at $z > 8$ , newly discovered by JWST/NIRSpec in the JADES survey. These three similarly bright ($M_{\mathrm{UV}} \approx -20\ \mathrm{mag}$ ) LAEs exhibit small $\mathrm{Ly}\alpha$ velocity offsets from the systemic redshift, $\Delta v \leq 200\ \mathrm{km/s}$ , yet span a range of $\mathrm{Ly}\alpha$ equivalent widths ($15\ \mathrm{\AA}$ , $31\ \mathrm{\AA}$ , and $132\ \mathrm{\AA}$ ). The former two show moderate $\mathrm{Ly}\alpha$ escape fractions ($f_{\mathrm{esc},\,\mathrm{Ly}\alpha} \approx 10\%$ ), whereas $\mathrm{Ly}\alpha$ escapes remarkably efficiently from the third ($f_{\mathrm{esc},\,\mathrm{Ly}\alpha} \approx 71\%$ ), which moreover is very compact (half-light radius of $90 \pm 10\ \mathrm{pc}$ ). We find these LAEs are low-mass galaxies dominated by very recent, vigorous bursts of star formation accompanied by strong nebular emission from metal-poor gas. We infer the two LAEs with modest $f_{\mathrm{esc},\,\mathrm{Ly}\alpha}$ , one of which reveals evidence for ionisation by an active galactic nucleus, may have reasonably produced small ionised bubbles preventing complete IGM absorption of $\mathrm{Ly}\alpha$ . The third, however, requires a $\sim 3\ \mathrm{pMpc}$ bubble, indicating faint galaxies have contributed significantly. The most distant LAEs thus continue to be powerful observational probes into the earlier stages of reionisation.

eprint arXiv:2404.05724