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