BEAGLE-AGN I: simultaneous constraints on the properties of gas in star-forming and AGN narrow-line regions in galaxies

We present the addition of nebular emission from the narrow-line regions (NLR) surrounding active galactic nuclei (AGNs) to BEAGLE (BayEsian Analysis of GaLaxy sEds). Using a set of idealized spectra, we fit to a set of observables (emission-line ratios and fluxes) and test the retrieval of different physical parameters. We find that fitting to standard diagnostic-line ratios plus [O II] λ3726, λ3729/[O III] λ5007, H β/H α, [O I] λ6300/[O II] λ3726, λ3729, and H α flux, degeneracies remain between dust-to-metal mass ratio ($\xi _\rm {d}^{\small NLR}$) and ionization parameter ($U_\rm {s}^{\small NLR}$) in the NLR gas, and between slope of the ionizing radiation ($\alpha _{\small PL}$, characterizing the emission from the accretion disc around the central black hole) and total accretion-disc luminosity ($L_\rm {acc}$). Since these degeneracies bias the retrieval of other parameters even at maximal signal-to-noise ratio (S/N), without additional observables, we suggest fixing $\alpha _{\small PL}$ and dust-to-metal mass ratios in both NLR and H II regions. We explore the S/N in H β required for un-biased estimates of physical parameters, finding that S/N(H β) ~ 10 is sufficient to identify an NLR contribution, but that higher S/N is required for un-biased parameter retrieval (~20 for NLR-dominated systems, ~30 for objects with approximately equal H β contributions from NLR and H II regions). We also compare the predictions of our models for different line ratios to previously published models and data. By adding [He II] λ4686-line measurements to a set of published line fluxes for a sample of 463 AGN NLR, we show that our models with $-4\lt \hbox{$\log U_{\small S}^{\small NLR}$}\lt -1.5$ can account for the full range of observed AGN properties in the local Universe.