Type 1 AGN at low z - II. The relative strength of narrow lines and the nature of intermediate type AGN

We explore the relative strength of the narrow emission lines in a Sloan Digital Sky Survey based sample of broad Hα selected active galactic nuclei (AGN), defined in Paper I. We find a decrease in the narrow to broad Hα luminosity (L_bHα) ratio with increasing L_bHα, such that both L([O iii ] λ5007) and L( narrow Hα) scale as ∝L bH α0.7 for 1040<L bH α<1045 erg s-1. Following our earlier result that L_bHα ∝ L_bol, this trend indicates that the relative narrow line luminosity decreases with increasing L_bol. We derive L bol /1043 erg s-1=4000(L([O iii ])/1043 erg s-1)1.39. This implies that the bolometric correction factor, L bol /L([O iii ]), decreases from 3000 at L bol =1046.1 erg s-1 to 300 at L bol =1042.5 erg s-1. At low luminosity, the narrow component dominates the observed Hα profile, and most type 1 AGN appear as intermediate type AGN. Partial obscuration or extinction cannot explain the dominance of intermediate type AGN at low luminosity, and the most likely mechanism is a decrease in the narrow line region covering factor with increasing L_bol. Deviations from the above trend occur in objects with L/L_Edd ≲ 10^-2.6, probably due to the transition to LINERs with suppressed [O III] emission, and in objects with M_BH > 10^8.5 M_⊙, probably due to the dominance of radio-loud AGN, and associated enhanced [O III] emission.