2012MNRAS.426.2703S

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([OIII]λ5007) and L(nHα) scale as ∝L(bHα)^0.7 for 10⁴⁰<L(bHα)<10⁴⁵erg/s. 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/10⁴³erg/s = 4000(L([OIII])/10⁴³erg/s)^1.39. This implies that the bolometric correction factor, L_bol/L([OIII]), decreases from 3000 at L_bol=10^46.1erg/s to 300 at L_bol=10^42.5erg/s. 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 [OIII] emission, and in objects with M_BH>10^8.5M_☉, probably due to the dominance of radio-loud AGN, and associated enhanced [OIII] emission.