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

We report the discovery of a remarkable Ly$α$ emitting galaxy at z = 7.2782, JADES-GS-z7-LA, with EW₀(Ly$α$) = 388.0 ± 88.8Åand UV magnitude -17.0. The spectroscopic redshift is confirmed via rest-frame optical lines [O II], H$β$ and [O III] in its JWST/NIRSpec Micro-Shutter Assembly (MSA) spectrum. The Ly$α$ line is detected in both lower resolution PRISM as well as medium resolution G140M grating spectra. The LSF-deconvolved Ly$α$ FWHM in the grating is 383.9 ± 56.2 km/s and the Ly$α$ velocity offset compared to the systemic redshift is 113.3 ± 80.0 km/s, indicative of very little neutral gas or dust within the galaxy. We estimate the Ly$α$ escape fraction to be >70%. JADES-GS-z7-LA has a O32 ratio of 11.1 ± 2.2 and a R23 ratio of 11.2 ± 2.6, 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$β$ 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 ≈ 10⁷ M_. Assuming that the Ly$α$ transmission through the intergalactic medium is the same as its measured escape fraction, an ionized region of size > 1.5 pMpc is needed to explain the high Ly$α$ EW and low velocity offset compared to systemic seen in JADES-GS-z7-LA. Owing to its UV-faintness, we show that it is incapable of single-handedly ionizing a region large enough to explain its Ly$α$ emission. 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 $z>7$.