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

TL;DR

This study analyzes optical narrow line ratios in a large sample of low-redshift type 1 AGN, revealing their relation to host galaxy properties, star formation, and metallicity, and comparing them with type 2 AGN.

Contribution

It provides a comprehensive analysis of narrow line ratios in type 1 AGN, exploring their dependence on host properties and comparing with type 2 AGN, highlighting selection effects and metallicity relations.

Findings

Narrow line luminosities follow a Baldwin relation with broad Hα luminosity.

20% of AGN show star formation signatures in BPT diagrams.

Type 1 AGN have lower [SII]/Ha and [NII]/Ha ratios than type 2 AGN, due to selection effects.

Abstract

We present the optical narrow line ratios in an SDSS based sample of 3,175 broad Ha selected type 1 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 (Ha, Hb, [OIII], [NII], [SII], [OI]) show a Baldwin relation relative to the broad Ha luminosity L_bHa, with slopes in the range of 0.53-0.72. 2. About 20% of the type 1 AGN reside within the `Composite' and `SF' regions of the BPT diagrams. These objects also show excess narrow Ha and UV luminosities, for their L_bHa, 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 [SII]/Ha and [NII]/Ha 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 [NII]/Ha and [NII]/[SII] 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. 5. The broad lines optical FeII is higher for a higher [NII]/Ha, 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 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 the extincting dust resides on scales larger than the NLR.