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

Abstract

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 () and ionization parameter () in the NLR gas, and between slope of the ionizing radiation (, characterizing the emission from the accretion disc around the central black hole) and total accretion-disc luminosity (). Since these degeneracies bias the retrieval of other parameters even at maximal signal-to-noise ratio (S/N), without additional observables, we suggest fixing 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 can account for the full range of observed AGN properties in the local Universe. © 2023 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society.