Interrogating Seyferts with NebulaBayes: Spatially probing the narrow-line region radiation fields and chemical abundances

TL;DR

NebulaBayes is a Bayesian tool for spatially resolving chemical abundances and radiation fields in the narrow-line regions of active galaxies, revealing metallicity gradients and ionization properties.

Contribution

We introduce NebulaBayes, a novel Bayesian code that compares observed emission lines to photoionization models for detailed spatial analysis of AGN narrow-line regions.

Findings

Constant ionization parameters across ENLRs.

Steep radial metallicity gradients in some galaxies.

Evidence of gas inflow or mergers affecting metallicity.

Abstract

NebulaBayes is a new Bayesian code that implements a general method of comparing observed emission-line fluxes to photoionization model grids. The code enables us to extract robust, spatially resolved measurements of abundances in the extended narrow line regions (ENLRs) produced by Active Galactic Nuclei (AGN). We observe near-constant ionization parameters but steeply radially-declining pressures, which together imply that radiation pressure regulates the ENLR density structure on large scales. Our sample includes four `pure Seyfert' galaxies from the S7 survey that have extensive ENLRs. NGC2992 shows steep metallicity gradients from the nucleus into the ionization cones. An {\it inverse} metallicity gradient is observed in ESO138-G01, which we attribute to a recent gas inflow or minor merger. A uniformly high metallicity and hard ionizing continuum are inferred across the ENLR of Mrk573. Our analysis of IC5063 is likely affected by contamination from shock excitation, which appears to soften the inferred ionizing spectrum. The peak of the ionizing continuum E_peak is determined by the nuclear spectrum and the absorbing column between the nucleus and the ionized nebula. We cannot separate variation in this intrinsic E_peak from the effects of shock or HII region contamination, but E_peak measurements nevertheless give insights into ENLR excitation. We demonstrate the general applicability of NebulaBayes by analyzing a nuclear spectrum from the non-active galaxy NGC4691 using a HII region grid. The NLR and HII region model grids are provided with NebulaBayes for use by the astronomical community.