Mixing between Seyfert and H ii region excitation in local active galaxies

We fit theoretical models to the emission-line spectra of 2766 Seyfert galaxies from the Sloan Digital Sky Survey (SDSS). The model line fluxes are derived by "mixing" photoionization model predictions of active galactic nucleus (AGN) narrow line region (NLR) emission and H ii region emission. The observed line fluxes in each spectrum were directly compared to the grid of mixed models using the Bayesian parameter estimation code NebulaBayes, thereby measuring the degree of mixing in each spectrum for the first time. We find that the majority of the Balmer line emission in the majority of Seyfert-classified SDSS spectra arises from contaminating H ii regions within the fixed-size aperture. Even for spectra with log [O iii]/Hβ ≈ 0.9, a median of ∼30% of the Balmer flux arises in H ii regions. We show that the results are qualitatively insensitive to the assumed Seyfert ionizing continuum, and that ionizing spectra with a peak energy of E peak ∼ 40-50 eV produce the most plausible distributions of mixing fractions. The analysis cleanly quantifies how the starburst - AGN "mixing fraction" increases on the Baldwin, Phillips, & Terlevich (BPT) diagram for SDSS galaxies. Apart from the mixing fraction, the models also vary with the ionization parameter in the NLR, the gas pressure, and the metallicity. Measurements for the metallicity in particular will be presented in a companion paper.