JADES: Discovery of extremely high equivalent width Lyman-α emission from a faint galaxy within an ionized bubble at z = 7.3
Chevallard, Jacopo; Maseda, Michael V.; Carniani, Stefano; Übler, Hannah; D'Eugenio, Francesco; Arribas, Santiago; Bunker, Andrew J.; Perna, Michele; Cameron, Alex J.; Jones, Gareth C.; Kumari, Nimisha; Scholtz, Jan; Willott, Chris; Johnson, Benjamin D.; Ji, Zhiyuan; Shivaei, Irene; Nelson, Erica; Chen, Zuyi; Endsley, Ryan; Rix, Hans-Walter; Saxena, Aayush; Curtis-Lake, Emma; Jakobsen, Peter; Smit, Renske; Witstok, Joris; Curti, Mirko; Ferruit, Pierre; Charlot, Stephane; Looser, Tobias J.; Rawle, Tim; Del Pino, Bruno Rodríguez; Egami, Eiichi; Eisenstein, Daniel J.; Hainline, Kevin; Hausen, Ryan; Rieke, Marcia; Robertson, Brant E.; Tacchella, Sandro; Williams, Christina C.; Willmer, Christopher N. A.; Baker, William M.; Bhatawdekar, Rachana; Bowler, Rebecca; Boyett, Kristan; Sandles, Lester; Simmonds, Charlotte; De Graaff, Anna
United Kingdom, United States, France, Canada, Italy, Spain, Denmark, Germany, Netherlands, Australia
Abstract
We report the discovery of a remarkable Lyα emitting galaxy at z = 7.2782, JADES-GS+53.16746−27.7720 (shortened to JADES-GS-z7-LA), with rest-frame equivalent width, EW0(Lyα) = 388.0 ± 88.8 Å and UV magnitude −17.0. The spectroscopic redshift is confirmed via rest-frame optical lines [O II], Hβ and [O III] in its JWST/NIRSpec Micro-Shutter Assembly (MSA) spectrum. The Lyα line is detected in both lower resolution (R ∼ 100) PRISM as well as medium resolution (R ∼ 1000) G140M grating spectra. The line spread function-deconvolved Lyα full width at half maximum in the grating is 383.9 ± 56.2 km s−1 and the Lyα velocity offset compared to the systemic redshift is 113.3 ± 80.0 km s−1, indicative of very little neutral gas or dust within the galaxy. We estimate the Lyα escape fraction to be > 70%. JADES-GS-z7-LA has a [O III]/[O II] ratio (O32) of 11.1 ± 2.2 and a ([O III] + [O II])/Hβ ratio (R23) 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] + Hβ 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 Lyα 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 Lyα 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 Lyα 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.