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

¹ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
e-mail: aayush.saxena@physics.ox.ac.uk
² Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
³ Department of Astronomy and Astrophysics, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA
⁴ Department of Astronomy, University of Texas, Austin, TX 78712, USA
⁵ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis Bd Arago, 75014 Paris, France
⁶ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁷ Cavendish Laboratory, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK
⁸ NRC Herzberg, 5071 West Saanich Rd, Victoria, BC V9E 2E7, Canada
⁹ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
¹⁰ Center for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, Hatfield AL10 9AB, UK
¹¹ European Space Agency, European Space Astronomy center, Camino Bajo del Castillo s/n, 28692 Villafranca del Castillo, Madrid, Spain
¹² Cosmic Dawn Center (DAWN), Copenhagen, Denmark
¹³ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen, Denmark
¹⁴ Centro de Astrobiología (CAB), CSIC-INTA, Cra. de Ajalvir Km. 4, 28850 Torrejón de Ardoz, Madrid, Spain
¹⁵ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei Muenchen, Germany
¹⁶ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁷ Department of Astronomy, University of Wisconsin-Madison, 475 N. Charter St., Madison, WI 53706, USA
¹⁸ European Space Agency, Space Telescope Science Institute, Baltimore, MA, USA
¹⁹ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
²⁰ Centre for Astrophysics, Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
²¹ Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA
²² Department of Physics and Astronomy, The Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218, USA
²³ NSF’s National Optical-Infrared Astronomy Research Laboratory, 950 North Cherry Avenue, Tucson, AZ 85719, USA
²⁴ European Space Agency, ESA/ESTEC, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
²⁵ Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, School of Natural Sciences, The University of Manchester, Manchester M13 9PL, UK
²⁶ School of Physics, University of Melbourne, Parkville, 3010 VIC, Australia
²⁷ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Stromlo, ACT 2611, Australia
²⁸ AURA for European Space Agency, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21210, USA
²⁹ Department for Astrophysical and Planetary Science, University of Colorado, Boulder, CO 80309, USA

Received: 24 February 2023
Accepted: 29 August 2023

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.