Article,
The host galaxies and classification of active galactic nuclei
Monthly Notices of the Royal Astronomical Society, 372 (3): 961--976 (2006)
DOI: 10.1111/j.1365-2966.2006.10859.x
Abstract
We present an analysis of the host properties of 85 224 emission-line galaxies selected from the Sloan Digital Sky Survey. We show that Seyferts and low-ionization narrow emission-line regions (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 active galactic nucleus–H ii (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, less concentrated, and they have higher velocity dispersions and lower O iii luminosities than Seyfert galaxies have. Seyferts and LINERs are most strongly distinguished by their O iii luminosities. We then consider the quantity LO iii/σ4, which is an indicator of the black hole accretion rate relative to the Eddington rate. Remarkably, we find that at fixedLO iii/σ4, 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 models and show that pure LINERs require a harder ionizing radiation field with lower ionization parameter than required by Seyfert galaxies, consistent with the low and high X-ray binary states.
