Inside the bubble: exploring the environments of reionisation-era Lyman-alpha emitting galaxies with JADES and FRESCO

We present a study of the environments of $17$ $\mathrm{Lyman}-\alpha$ emitting galaxies (LAEs) in the reionisation-era ($5.8 < z < 8$ ) identified by JWST/NIRSpec as part of the JWST Advanced Deep Extragalactic Survey (JADES). Unless situated in sufficiently (re)ionised regions, $\mathrm{Lyman}-\alpha$ emission from these galaxies would be strongly absorbed by neutral gas in the intergalactic medium (IGM). We conservatively estimate sizes of the ionised regions required to reconcile the relatively low $\mathrm{Lyman}-\alpha$ velocity offsets ($\Delta v_{\mathrm{Ly}\alpha} < 300\ \mathrm{km/s}$ ) with moderately high $\mathrm{Lyman}-\alpha$ escape fractions ($f_{\mathrm{esc},\,\mathrm{Ly}\alpha} > 5\%$ ) observed in our sample of LAEs, suggesting the presence of ionised hydrogen along the line of sight towards at least eight out of $17$ LAEs. We find minimum physical “bubble” sizes of the order of $R_{\mathrm{ion}} \sim 0.1-1\ \mathrm{pMpc}$ are required in a patchy reionisation scenario where ionised bubbles containing the LAEs are embedded in a fully neutral IGM. Around half of the LAEs in our sample are found to coincide with large-scale galaxy overdensities seen in FRESCO at $z \sim 5.8-5.9$ and $z \sim 7.3$ , suggesting $\mathrm{Lyman}-\alpha$ transmission is strongly enhanced in such overdense regions, and underlining the importance of LAEs as tracers of the first large-scale ionised bubbles. Considering only spectroscopically confirmed galaxies, we find our sample of UV-faint LAEs ($M_{\mathrm{UV}} \geq -20\ \mathrm{mag}$ ) and their direct neighbours are generally not able to produce the required ionised regions based on the $\mathrm{Lyman}-\alpha$ transmission properties, suggesting lower-luminosity sources likely play an important role in carving out these bubbles. These observations demonstrate the combined power of JWST multi-object and slitless spectroscopy in acquiring a unique view of the early Universe during cosmic reionisation via the most distant LAEs.

Astronomy & Astrophysics, Volume 682, id.A40, 15 pages