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

Joris Witstok, Renske Smit, Aayush Saxena, Gareth C. Jones, Jakob M. Helton, Fengwu Sun, Roberto Maiolino, Daniel P. Stark, Andrew J. Bunker, Santiago Arribas, William M. Baker, Rachana Bhatawdekar, Kristan Boyett, Alex J. Cameron, Stefano Carniani, Stephane Charlot, Jacopo Chevallard, Mirko Curti, Emma Curtis-Lake, Daniel J. Eisenstein, Ryan Endsley, Kevin Hainline, Zhiyuan Ji, Benjamin D. Johnson, Nimisha Kumari, Tobias J. Looser, Erica Nelson, Michele Perna, Hans-Walter Rix, Brant E. Robertson, Lester Sandles, Jan Scholtz, Charlotte Simmonds, Sandro Tacchella, Hannah Übler, Christina C. Williams, Christopher N. A. Willmer, Chris Willott

Published 2023-06-07Version 1

We present a study of the environments of 16 Lyman-$\alpha$ emitting galaxies (LAEs) in the reionisation era ($5.8 < z < 8$) identified by $\mathit{JWST}$/NIRSpec as part of the $\mathit{JWST}$ Advanced Deep Extragalactic Survey (JADES). Unless situated in sufficiently (re)ionised regions, 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 Lyman-$\alpha$ velocity offsets ($\Delta v_\mathrm{Ly\alpha} < 300 \, \mathrm{km \, s^{-1}}$) with moderately high Lyman-$\alpha$ escape fractions ($f_\mathrm{esc, \, Ly\alpha} > 5\%$) observed in our sample of LAEs, indicating the presence of ionised ``bubbles'' with physical sizes of the order of $0.1 \, \mathrm{pMpc} \lesssim R_\text{ion} \lesssim 1 \, \mathrm{pMpc}$ in a patchy reionisation scenario where the bubbles 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 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 ($M_\text{UV} \gtrsim -20 \, \mathrm{mag}$) and their direct neighbours are generally not able to produce the required ionised regions based on the 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 $\mathit{JWST}$ multi-object and slitless spectroscopy in acquiring a unique view of the early stages of Cosmic Reionisation via the most distant LAEs.