Uncovering the physical origin of the prominent Lyman-α emission and absorption in GS9422 at z = 5.943

¹ Cosmic Dawn Center (DAWN), Rådmandsgade 64, 2200 Copenhagen N, Denmark
² Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen N, Denmark
³ Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁴ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
⁵ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁶ Cavendish Laboratory, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK
⁷ Astrophysics Research Institute, Liverpool John Moores University, Liverpool L35 UG, UK

Received: 15 April 2024
Accepted: 2 August 2024

Abstract

We present a comprehensive spectrophotometric analysis of galaxy GS9422 from the JADES GTO survey located at z = 5.943, anomalously showing a simultaneous strong Lyα emission feature and damped Lyα absorption (DLA), based on JWST NIRSpec and NIRCam observations. The best-fit modeling of the spectral energy distribution (SED) reveals a young, low-mass (log(M_⋆/M_⊙) = 7.80 ± 0.01) galaxy, with a mass-weighted mean age of the stellar population of (10.90_−0.12^+0.07) Myr. The identified strong nebular emission lines suggest a highly ionized (O₃₂ = 59), low-metallicity (12 + log(O/H) = 7.78 ± 0.10) star-forming galaxy with a star-formation rate (SFR) of (8.2 ± 2.8) M_⊙ yr⁻¹ over a compact surface area A_e = 1.85 kpc², typical for galaxies at this epoch. This corresponds to an intense SFR surface density of log(Σ_SFR/M_⊙ yr⁻¹ kpc⁻²) = 1.14 ± 0.30. We carefully modeled the rest-frame UV NIRSpec Prism spectrum around the Lyα edge, finding that the Lyα emission-line redshift is consistent with the longer-wavelength recombination lines and an escape fraction of f_esc, Lyα = 30% but that the broad DLA feature is not able to converge on the same redshift. Instead, our modeling suggests z_abs = 5.40 ± 0.10, the exact redshift of a newly identified protocluster in nearby projection to the target galaxy. We argue that most of the H I gas producing the strong Lyα damping wing must be unassociated with the galaxy itself, and thus may indicate that we are probing the cold, dense circumcluster medium of this massive galaxy overdensity. These results provide an alternative solution to the recent claims of continuum nebular emission or an obscured active galactic nucleus dominating the rest-frame UV parts of the spectrum and provide further indications that strong DLAs might preferentially be associated with galaxy overdensities.