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

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 Ly α emission feature and damped Ly α 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 (O 32 = 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 kpc 2 , 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 Ly α edge, finding that the Ly α emission-line redshift is consistent with the longer-wavelength recombination lines and an escape fraction of f esc, Ly α = 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 Ly α 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.

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