JADES: Primaeval Lyman-$\alpha$ emitting galaxies reveal early sites of reionisation out to redshift $z\sim 9$

Abstract

$\require{mediawiki-texvc}$Given the sensitivity of the resonant Lyman-$\alpha$ (Ly$\alpha$) transition to absorption by neutral hydrogen, observations of 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 ($M_{\text{UV}}\approx -20\mathrm{mag}$) LAEs exhibit small Ly$\alpha$ velocity offsets from the systemic redshift, $\Delta v_{\mathrm{Ly}\alpha }\lesssim 200\mathrm{km}{\mathrm{s}}^{-1}$, yet span a range of Ly$\alpha$ equivalent widths ($15\mathring{A}$, $31\mathring{A}$, and $132\mathring{A}$). The former two show moderate Ly$\alpha$ escape fractions ($f_{\mathrm{esc},\mathrm{Ly}\alpha }\approx 10\%$), whereas Ly$\alpha$ escapes remarkably efficiently from the third ($f_{\mathrm{esc},\mathrm{Ly}\alpha }\approx 72\%$), 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 Ly$\alpha$. The third, however, requires a $\sim 3\text{physical Mpc}$ bubble, indicating faint galaxies have contributed significantly. The most distant LAEs thus continue to be powerful observational probes into the earlier stages of reionisation.

25 pages, 13 figures, accepted for publication in MNRAS

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