JADES: primaeval Lyman α emitting galaxies reveal early sites of reionization out to redshift z ~ 9

Abstract

Given the sensitivity of the resonant Lyman-α (Lyα) transition to absorption by neutral hydrogen, observations of Lyα 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 (MUV ≈ −20 mag) LAEs exhibit small Lyα velocity offsets from the systemic redshift, ΔvLy$α$ ≲ 200 km s−1, yet span a range of Lyα equivalent widths (15 Å, 31 Å, and 132 Å). The former two show moderate Lyα escape fractions ($f_\text{esc, {Ly$\alpha $}} \approx 10\%$), whereas Lyα escapes remarkably efficiently from the third ($f_\text{esc, {Ly$\alpha $}} \approx 72\%$), which moreover is very compact (half-light radius of 90 ± 10 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 fesc, Ly$α$, one of which reveals evidence for ionisation by an active galactic nucleus, may have reasonably produced small ionised bubbles preventing complete IGM absorption of Lyα. The third, however, requires a ∼3 physicalMpc bubble, indicating faint galaxies have contributed significantly. The most distant LAEs thus continue to be powerful observational probes into the earlier stages of reionisation.