JADES: Discovery of extremely high equivalent width Lyman-alpha emission from a faint galaxy within an ionized bubble at $z=7.3$

Aayush Saxena, Brant E. Robertson, Andrew J. Bunker, Ryan Endsley, Alex J. Cameron, Stephane Charlot, Charlotte Simmonds, Sandro Tacchella, Joris Witstok, Chris Willott, Stefano Carniani, Emma Curtis-Lake, Pierre Ferruit, Peter Jakobsen, Santiago Arribas, Jacopo Chevallard, Mirko Curti, Francesco D'Eugenio, Anna De Graaff, Gareth C. Jones, Tobias J. Looser, Michael V. Maseda, Tim Rawle, Hans-Walter Rix, Bruno Rodr\'{iguez Del Pino, Renske Smit, Hannah Übler, Daniel J. Eisenstein, Kevin Hainline, Ryan Hausen, Benjamin D. Johnson, Marcia Rieke, Christina C. Williams, Christopher N. A. Willmer, William M. Baker, Rachana Bhatawdekar, Rebecca Bowler, Kristan Boyett, Zuyi Chen, Eiichi Egami, Zhiyuan Ji, Erica Nelson, Michele Perna, Lester Sandles, Jan Scholtz, Irene Shivaei

Published 2023-02-24Version 1

We report the discovery of a remarkable Ly$\alpha$ emitting galaxy at $z=7.278$, JADES-GS+53.16746-27.7720 (shortened to JADES-GS-z7-LA), with EW$_0$(Ly$\alpha$) $\approx400 \pm 90$A and UV magnitude $-16.7$. The spectroscopic redshift is confirmed via rest-frame optical lines [O II], H$\beta$ and [O III] in its JWST/NIRSpec Micro-Shutter Assembly (MSA) spectrum. The Ly$\alpha$ line is detected in both lower resolution ($R\sim100$) PRISM as well as medium resolution ($R\sim1000$) G140M grating spectra. The Ly$\alpha$ FWHM in the grating is $\approx360$ km s$^{-1}$ and the line peaks within $120$ km s$^{-1}$ of the systemic redshift, indicative of very little neutral gas or dust within the galaxy. We estimate the Ly$\alpha$ escape fraction to be $\sim100\%$. JADES-GS-z7-LA has a [O III]/[O II] ratio (O32) of $8.8 \pm 1.1$ and ([O III]+[O II])/H$\beta$ ratio (R23) of $9.6\pm2.2$, consistent with low metallicity and high ionization parameters. Deep NIRCam imaging also revealed a close companion source (separated by $0.23''$), which exhibits similar photometry to that of JADES-GS-z7-LA, with a photometric excess in the F410M NIRCam image consistent with [O III]+H$\beta$ emission at the same redshift. The spectral energy distribution of JADES-GS-z7-LA indicates a `bursty' star-formation history, with a low stellar mass of $10^{7.15}$ M$_\odot$. The only explanation of the high EW Ly$\alpha$ emission seen in JADES-GS-z7-LA is if it resides in an ionized bubble with radius $\gtrsim 3$ pMpc. Owing to the faint nature of JADES-GS-z7-LA, we show that it is incapable of single-handedly ionizing a bubble large enough. Therefore, we suggest that JADES-GS-z7-LA (and possibly the companion source) may be a part of a larger overdensity, presenting direct evidence of overlapping ionized bubbles at $z>7$, allowing us to study the process of reionization across both small and large scales.