Uncovering the physical origin of the prominent Lyman-$α$ emission and absorption in GS9422 at $z = 5.943$ (2404.06543v1)

Abstract: 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 Ly$\alpha$ emission feature and damped Ly$\alpha$ absorption (DLA), based on JWST NIRSpec and NIRCam observations. The best-fit modelling of the spectral energy distribution (SED) reveals a young, low-mass (${\rm log}(M_\star/M_{\odot}) = 7.8 \pm 0.01$) 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 ($12+\log({\rm O/H}) = 7.78\pm 0.10$) star-forming galaxy with a star-formation rate SFR = ($8.2 \pm 2.8$) $\rm M_{\odot}\;yr^{-1}$ 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 Ly$\alpha$ edge, finding that the Ly$\alpha$ emission-line redshift is consistent with the longer-wavelength recombination lines and an escape fraction of $f_{\rm esc,Ly\alpha} = 30\%$ but that the broad DLA feature is not able to converge on the same redshift. Instead, our modelling suggests $z_{\rm abs}= 5.40 \pm 0.10$, 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 Ly$\alpha$ 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]

• The paper reveals that GS9422 is a young, low-mass, and metal-poor galaxy with a mass-weighted age of ~10.9 Myr and an SFR of ~8.2 M⊙/yr.
• The paper demonstrates that JWST data uncovers a strong Lyman-α emission (30% escape fraction) alongside a broad damped Lyman-α absorption likely linked to a nearby proto-cluster's intergalactic medium.
• The paper implies that the coexistence of Lyman-α emission and absorption features necessitates a reassessment of gas distribution and dynamics in high-redshift galaxy environments.

Analysis of Lyman-α Emission and Absorption in GS9422 at Redshift 5.943

This paper presents a detailed spectro-photometric analysis of the galaxy GS9422, identified in the JADES GTO survey and located at redshift $z = 5.943$. This galaxy exhibits both a prominent Lyman-α (Lyα) emission line and a damped Lyα absorption feature (DLA), observed using the Near InfraRed Spectrograph (NIRSpec) and Near InfraRed Camera (NIRCam) aboard the James Webb Space Telescope (JWST). The analysis focuses on elucidating the physical origins of these observed features within the context of high-redshift galaxies.

Key Findings

1. Physical Characteristics of GS9422:
   • The galaxy is characterized by its young stellar population and low stellar mass, with a mass-weighted mean age of approximately 10.9 Myr and a stellar mass $\log(M_\star/M_{\odot}) = 7.8 \pm 0.01$.
   • GS9422 is a highly ionized, low-metallicity galaxy, specified by a metallicity $\rm 12+\log(O/H) = 7.78\pm 0.10$ and an oxygen ionization parameter $O_{32} = 59$. The star formation rate (SFR) is measured at $(8.2 \pm 2.8) \, \rm M_{\odot}\, yr^{-1}$.
2. Lyman-α Emission and Absorption Analysis:
   • The Lyα emission line appears strong, with an escape fraction of $f_{\rm esc,Ly\alpha} = 30\%$. However, the broad DLA feature presents a modeling challenge as it cannot converge on the same redshift as the Lyα emission.
   • The paper suggests that the majority of the neutral hydrogen gas causing the strong DLA is possibly unassociated with GS9422. Instead, it may pertain to the dense intergalactic medium of a nearby galaxy proto-cluster at $z_{\rm abs} = 5.40 \pm 0.10$.
3. Astrophysical Implications:
   • The coexistence of strong Lyα emission and DLA suggests that during this epoch, significant dense neutral hydrogen masses might form around galaxy overdensities. This prompts reevaluation of previous interpretations attributing the spectral shape to obscured active galactic nucleus (AGN) or nebular emissions.

Theoretical and Practical Implications

The findings challenge the previously held assumptions regarding the spatial association between Lyα emission sources and the regions producing DLA features. The hypothesis of GS9422 probing a circumcluster medium introduces new dimensions to understanding the interactions between early galaxies and their surroundings. From a theoretical perspective, this implies a reconsideration of gas distribution and dynamics within proto-cluster environments at high redshifts.

Practically, this research underscores the necessity for refined spectroscopic analysis of high-redshift cosmic structures to disentangle absorption contributions from foreground intervenors. Continuous observations with JWST, supported by complementary ALMA data, could validate the existence of wide-spanning hydrogen reservoirs tracing cosmic structures.

Future Outlook

Continued exploration of galaxies exhibiting Lyα anomalies, as demonstrated by GS9422, will advance our comprehension of the complex scenarios contributing to the IGM's ionization history. Further observational campaigns targeting different sightlines across various redshift ranges would provide the statistical robustness needed to ascertain the nature of these absorptive features conclusively. Addressing these issues will enhance our understanding of galaxy evolution processes across cosmic time, specifically within the framework of gas accretion and feedback in the early universe.