The Host Galaxies and Classification of Active Galactic Nuclei

We present an analysis of the host properties of 85224 emission-line galaxies selected from the Sloan Digital Sky Survey. We show that Seyferts and LINERs form clearly separated branches on the standard optical diagnostic diagrams. We derive a new empirical classification scheme which cleanly separates star-forming galaxies, composite AGN-H ii galaxies, Seyferts and LINERs and we study the host galaxy properties of these different classes of objects. LINERs are older, more massive, less dusty and more concentrated, and they and have higher velocity dispersions and lower [OIII] luminosities than Seyfert galaxies. Seyferts and LINERs are most strongly distinguished by their [OIII] luminosities. We then consider the quantity L[OIII]/σ, which is an indicator of the black hole accretion rate relative to the Eddington rate. Remarkably, we find that at fixed L[OIII]/σ, all differences between Seyfert and LINER host properties disappear. LINERs and Seyferts form a continuous sequence, with LINERs dominant at low L/LEDD and Seyferts dominant at high L/LEDD . These results suggest that the majority of LINERs are AGN and that the Seyfert/LINER dichotomy is analogous to the high/low-state transition for X-ray binary systems. We apply theoretical photo-ionization models and show that pure LINERs require a harder ionizing radiation field with lower ionization parameter than Seyfert galaxies, consistent with the low and high X-ray binary states.