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The physical conditions in the narrow-line regions (NLRs) of active galactic nuclei (AGN) may be constrained by comparing spectroscopic observations with photoionisation models. The key parameters are the gas-phase abundances, the ionisation parameter, the gas pressure, and the shape of the ionising spectrum. The first part of this thesis presents a model of the ionising continuum radiation in Seyfert AGN. This model OXAF preserves the physically-motivated shapes of existing theoretical models while simplifying them to a three-parameter form designed for photoionisation modelling. The second part of the thesis is the presentation of the full data release of the Siding Spring Southern Seyfert Spectroscopic Snapshot Survey (S7). Three-dimensional data cubes, two-dimensional emission-line fits, and other products are provided to the astronomical community, along with a large number of maps of gas kinematics across the S7 sample. In the third part of the thesis, a Bayesian code is presented that compares photoionisation model grids with observed emission line fluxes. This code NebulaBayes is agnostic to model parameters, dimensionality, and the chosen emission lines. Grids of MAPPINGS photoionisation models are calculated and used with NebulaBayes to infer physical parameters in two dimensions across four ’pure Seyfert’ galaxies selected from the S7 sample. The results are the first robust two-dimensional measurements of gas-phase metallicity in Seyfert NLRs, and show steep metallicity gradients into extraplanar ionisation cones in the case of edge-on galaxy NGC 2992, and an inverse metallicity gradient in the galaxy ESO 138-G01. The near-constant ionisation parameters suggest that radiation pressure regulates the density structure of NLRs on multi-kiloparsec scales. The ionising radiation is measured to be harder in ionisation cones than elsewhere, but the results are sensitive to spectral contamination. The fourth part of this thesis addresses mixing of NLR emission with emission from HII-regions in star-forming galaxies. A ’mixing’ grid (combining HII and NLR model grids) is coupled with a statistically powerful sample from the Sloan Digital Sky Survey (SDSS) to constrain the HII-AGN ’mixing fractions’ using NebulaBayes. It is found that for the majority of Seyfert-classified SDSS spectra, the majority of the Balmer emission arises in HII regions as opposed to NLRs. The final results presented in the thesis are the first systematic and robust measurements of the gas-phase metallicities of active galaxies in the SDSS. The metallicities associated with Seyfert-classified emission are shown to be consistently high and the SDSS AGN follow the upper locus of the SDSS stellar mass-metallicity relation. |
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