Kavli Affiliate: Roberto Maiolino
| First 5 Authors: Sandro Tacchella, William McClymont, Jan Scholtz, Roberto Maiolino, Xihan Ji
| Summary:
Understanding the sources that power nebular emission in high-redshift
galaxies is fundamentally important not only for shedding light onto the
drivers of reionisation, but to constrain stellar populations and the growth of
black holes. Here we focus on an individual object, GS9422, a galaxy at $z_{rm
spec}=5.943$ with exquisite data from the JADES and JEMS surveys, including
14-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-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-UV
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 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 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.
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