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

Journal article

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- ³), 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

Authors: Saxena, A; Robertson, BE; Bunker, AJ; Endsley, R; Cameron, AJ

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: EDP Sciences
• Journal: Astronomy & Astrophysics
• Volume: 678
• Pages: A68-A68
• Publication date: 2023-08-31
• Acceptance date: 2023-08-29
• DOI: 10.1051/0004-6361/202346245
• EISSN: 1432-0746
• ISSN: 0004-6361
• Language: English
• Keywords: Photometry (optics); Spectral resolution; Redshift; Galaxy; Equivalent width; Metallicity; Stars; Emission spectrum; Physics; Spectral line; Astronomy; Astrophysics