JADES: Discovery of extremely high equivalent width Lyman-<i>α</i> emission from a faint galaxy within an ionized bubble at <i>z</i> = 7.3
Aayush Saxena, Brant Robertson, Andrew J. Bunker, Ryan Endsley, Alex J. Cameron, S. Charlot, Charlotte Simmonds, Sandro Tacchella, Joris Witstok, Chris J. Willott, Stefano Carniani, Emma Curtis-Lake, Pierre Ferruit, P. 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 Rodríguez 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 Baker, Rachana Bhatawdekar, R. A. A. Bowler, Kristan Boyett, Zuyi Chen, Eiichi Egami, Zhiyuan Ji, Nimisha Kumari, Erica J. Nelson, Michele Perna, Lester Sandles, Jan Scholtz, Irene Shivaei
Abstract
We report the discovery of a remarkable Ly α emitting galaxy at z = 7.2782, JADES-GS+53.16746−27.7720 (shortened to JADES-GS-z7-LA), with rest-frame equivalent width, EW 0 (Ly α ) = 388.0 ± 88.8 Å and 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 ( R ∼ 100) PRISM as well as medium resolution ( R ∼ 1000) G 140 M grating spectra. The line spread function-deconvolved Ly α full width at half maximum in the grating is 383.9 ± 56.2 km s −1 and the Ly α 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 Ly α 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 ])/H β 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 F 410 M 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 ≈10 7 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.