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

We present a comprehensive spectro-photometric analysis of the 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 modelling of the spectral energy distribution (SED) reveals a young, low-mass (log(M_⋆/M_⊙) = 7.8 ± 0.01) galaxy, with a mass-weighted mean age of the stellar population of (10.9^+0.07_-0.12)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 = (8.2 ± 2.8) M_⊙ yr^-1 over a compact surface area A_e = 1.85 kpc^2, typical for galaxies at this epoch. We carefully model 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 modelling suggests z_ abs= 5.40 ± 0.10, the exact redshift of a newly identified proto-cluster in nearby projection to the target galaxy. We argue that most of the HI gas producing the strong Lyα damping wing indeed has to 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. [Abridged]