The host galaxies and classification of active galactic nuclei

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 L[O III]/σ 4 , which is an indicator of the black hole accretion rate relative to the Eddington rate. Remarkably, we find that at fixed L[O III]/σ 4 , all differences between Seyfert and LINER host properties disappear. LINERs and Seyferts form a continuous sequence, with LINERs dominant at low L/L EDD and Seyferts dominant at high L/L EDD . 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.

Dusty star-forming galaxies at high redshift

“…Although optical spectral line diagnostics can provide a valuable probe to AGN activity in galaxies via comparison of emission line ratios (e.g. Kewley et al, 2006;Juneau et al, 2011), DSFGs, even if they have optical spectral observations, often lack the high-quality, high-S/N spectra necessary for classification.…”

Starburst–AGN mixing – II. Optically selected active galaxies

“…The minimum 'distance' (in [N II]/Hα vs [O III]/Hβ space) of a galaxy (or spectrum) from the enriched end of the star-forming sequence can be quantified using the starforming distance (dSF ; Kewley et al 2006). The dSF metric and other variations on it have been used as relative indicators of the contribution of star formation and AGN activity to the line emission of AGN host galaxies.…”

“…Each galaxy is classified as either a) pure star-forming, b) composite, c) Seyfert, d) LINER, e) Seyfert-LINER intermediate object or f) ambiguous based on the diagnostic lines of Kewley et al (2001), Kauffmann et al (2003) and Kewley et al (2006). A full description of the combined classification scheme can be found in Kewley et al (2006).…”

“…The hardness of the stellar ionizing radiation field increases with decreasing metallicity, perhaps due to stellar rotation or the effects of metal opacity (see e.g. de Mink et al 2009;Levesque et al 2012;Eldridge & Stanway 2012 Kauffmann et al 2003 empirical upper bound to pure star-formation) can also be occupied by star-forming galaxies with a significant fraction of LINER-like emission (Kewley et al 2006) e.g. due to shock excitation or ionization by post asymptotic giant branch (pAGB) stars (e.g.…”

“…The [O III]/Hβ ratio is sensitive to the electron temperature and ionization parameter of the line-emitting gas, and the [N II]/Hα ratio is a tracer of metallicity due to the secondary nucleosynthetic production of nitrogen at high metallicities (see Kewley et al 2013a for further discussion). Pure starforming galaxies and AGN host galaxies occupy two distinct sequences on the [N II]/Hα vs. [O III]/Hβ diagnostic diagram (see e.g Kauffmann et al 2003;Kewley et al 2006Kewley et al , 2013a. The positions of H II regions along the starforming sequence are determined primarily by their chemical abundances (Dopita & Evans 1986;Dopita et al 2000).…”