The mass-metallicity relation of local active galaxies

We systematically measure the gas-phase metallicities and the mass-metallicity relation of a large sample of local active galaxies for the first time. Observed emission-line fluxes from the Sloan Digital Sky Survey (SDSS) are compared to a four-dimensional grid of photoionization models using the Bayesian parameter estimation code NebulaBayes. For the first time we take into account arbitrary mixing between HII region and narrow-line region (NLR) emission, and the models are also varied with metallicity, ionization parameter in the NLR, and the gas pressure. The active galactic nucleus (AGN) oxygen abundance is found to increase by $\Delta {\rm O/H} \sim 0.1$ dex as a function of host galaxy stellar mass over the range $10.1 < \log M_* / M_\odot < 11.3$. We also measure the metallicity and ionization parameter of 231000 star-forming galaxies for comparison with the sample of 7670 Seyfert 2 galaxies. A systematic offset in oxygen abundance of 0.09 dex is observed between the mass-metallicity relations of the star-forming and active galaxies. We investigate potential causes of the offset, including sample selection and the treatment in the models of diffuse ionized gas, pressure, and ionization parameter. We cannot identify the major cause(s), but suspect contributions due to deficiencies in modeling the ionizing spectra and the treatment of dust physics. Optical diagnostic diagrams are presented with the star-forming and Seyfert data colored by the inferred oxygen abundance, ionization parameter and gas pressure, clearly illustrating the trends in these quantities.