Characterising the z $\sim$ 7.66 Type-II AGN candidate SMACS S06355 using BEAGLE-AGN and JWST NIRSpec/NIRCam

Abstract

The presence of Active Galactic Nuclei (AGN) in low mass (Mstar $\lesssim$ $10^9$ Msun) galaxies at high redshift has been established, and it is important to characterise these objects and the impact of their feedback on the host galaxies. In this paper we apply the Spectral Energy Distribution (SED) fitting code BEAGLE-AGN to SMACS S06355, a z $\sim$ 7.66 Type-II AGN candidate from the JWST NIRSpec Early Release Observations. This object's spectrum includes a detection of the [NeIV]2426 line, indicating an obscured AGN due to its high ionization potential energy ($\sim$ 63eV). We use BEAGLE-AGN to simultaneously model the Narrow Line Region (NLR) AGN and star-forming galaxy contributions to the observed line fluxes and photometry. Having a high-ionization emission line allows the contribution of the NLR to the remaining lines to be probabilistically disentangled. The HII region metallicity is derived to be 12+log(O/H)${}^{\mathrm{HII}}$ = $7.82_{-0.19}^{+0.18}$. Assuming that the Neon-to-Oxygen abundance is similar to solar we derive a high NLR metallicity of 12+log(O/H)${}^{\mathrm{NLR}}$ = $8.86_{-0.16}^{+0.14}$, with the 2$\sigma$ lower-limit extending to 12+log(O/H)${}^{\mathrm{NLR}}$ $\sim$ 8.54, showing the derivation is uncertain. We discuss this result with respect to non-solar Neon abundances that might boost the inferred NLR metallicity. The NLR metallicity places SMACS S06355 in a comparable region of the mass-metallicity plane to intermediate (1.5 $\lesssim$ z $\lesssim$ 3.0) redshift obscured AGN. Our derived accretion disc luminosity, log($L_{acc}$ / erg $s^{-1}$) = $45.19_{-0.11}^{+0.12}$, is moderately high yet still uncertain. We highlight that deviations between bolometric luminosity calibrations and model grid tracks become enhanced at low metallicities.

16 pages, 9 figures

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