Type 1 AGN at low z - III. The optical narrow-line ratios

We present the optical narrow-line ratios in a Sloan Digital Sky Survey (SDSS) based sample of 3175 broad H alpha selected type 1 active galactic nuclei (AGN), and explore their positions in the BPT diagrams as a function of the AGN and the host properties. We find the following: (1) the luminosities of all measured narrow lines (H alpha, H beta, [OIII], [N II], [S II], [O I]) show a Baldwin relation relative to the broad H alpha luminosity L-bH alpha, with slopes in the range of 0.53-0.72. (2) About 20 per cent of the type 1 AGN reside within the 'Composite' and 'star-forming' (SF) regions of the Baldwin, Phillips & Terlevich (BPT) diagrams. These objects also show excess narrow H alpha and ultraviolet (UV) luminosities, for their L-bH alpha, consistent with contribution from star formation which dominates the narrow-lines emission, as expected from their positions in the BPT diagrams. (3) The type 1 which reside within the AGN region in the BPT diagrams, are offset to lower [S II]/H alpha and [N II]/H alpha luminosity ratios, compared to type 2 AGN. This offset is a selection effect, related to the lower AGN/host luminosity selection of the type 2 AGN selected from the SDSS galaxy sample. (4) The [N II]/H alpha and [N II]/[S II] ratios in type 1 AGN increase with the host mass, as expected if the mass-metallicity relation of quiescent galaxies holds for the AGN narrow-line region (NLR). (5) The broad lines optical Fe II is higher for a higher [N II]/H alpha, at a fixed L-bol and Eddington ratio L/L-Edd. This suggests that the broad line region metallicity is also related to the host mass. (6) The fraction of AGN which are low-ionization nuclear emission-line regions (LINERs) increases sharply with decreasing L/L-Edd. This fraction is the same for type 1 and type 2 AGN. (7) The BPT position is unaffected by the amount of dust extinction of the optical-UV continuum, which suggests that the extincting dust resides on scales larger than the NLR.