Resolving the nature and putative nebular emission of GS9422: an obscured AGN without exotic stars

Understanding the sources that power nebular emission in high-redshift galaxies is fundamentally important not only for shedding light onto the drivers of reionization, but to constrain stellar populations and the growth of black holes. Here we focus on an individual object, GS9422, a galaxy at $z_{\mathrm{spec}} = 5.943$ with exquisite data from the JWST Advanced Deep Extragalactic Survey (JADES), JWST Extragalactic Medium-band Survey (JEMS), and First Reionization Epoch Spectroscopically Complete Observations (FRESCO) surveys, including $14-\mathrm{band}$ JWST/NIRCam photometry and deep NIRSpec prism and grating spectroscopy. We map the continuum emission and nebular emission lines across the galaxy on $0.2-\mathrm{kpc}$ scales. GS9422 has been claimed to have nebular-dominated continuum and an extreme stellar population with top-heavy initial mass function. We find clear evidence for different morphologies in the emission lines, the rest-ultraviolet and rest-optical continuum emission, demonstrating that the full continuum cannot be dominated by nebular emission. While multiple models reproduce the spectrum reasonably well, our preferred model with a type-2 active galactic nucleus (AGN) and local damped $\mathrm{Ly}-\alpha$ (DLA) clouds can explain both the spectrum and the wavelength-dependent morphology. The AGN powers the off-planar nebular emission, giving rise to the Balmer jump and the emission lines, including $\mathrm{Ly}-\alpha$ , which therefore does not suffer DLA absorption. A central, young stellar component dominates the rest-UV emission and – together with the DLA clouds – leads to a spectral turn over. A disc-like, older stellar component explains the flattened morphology in the rest-optical continuum. We conclude that GS9422 is consistent with being a normal galaxy with an obscured, type-2 AGN – a simple scenario, without the need for exotic stellar populations.

Monthly Notices of the Royal Astronomical Society, Volume 540, Issue 1, pp. 851-870, 20 pages