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Disambiguate the excitation mechanism of the emission lines in RUBIES-EGS-QG-1 (AGN vs shocks)

Determine whether the elevated nebular line emission observed in RUBIES-EGS-QG-1—characterized by weak Hα emission and high [N II]λ6585/Hα and [O III]/Hβ ratios—is powered by an active galactic nucleus or by shocked gas, given that current data cannot exclude shock excitation.

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Background

The JWST/NIRSpec spectra reveal [O III], [S II], Hα, and [N II] emission in a galaxy whose continuum shows deep Balmer absorption and a strong 4000 Å break, indicating a quiescent/post-starburst system. The inferred line ratios exceed those expected from star formation, pointing toward non-stellar ionization sources.

Based on the diagnostics, the authors suggest an AGN is likely present; however, given the data quality and possible alternative ionization mechanisms, they explicitly note that shocked gas cannot be ruled out as the source of the emission.

References

Therefore, the emission lines do not appear to be connected to ongoing star formation activity, but rather suggest the presence of an active galactic nucleus (AGN), although we cannot rule out the presence of shocked gas.

— Efficient formation of a massive quiescent galaxy at redshift 4.9 (2404.05683 - Graaff et al., 8 Apr 2024) in Main text, spectral analysis paragraph describing emission lines (following Figure 1)