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

Abstract

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 (LbHα) ratio with increasing LbHα, such that both L([O iii]λ5007) and L( narrow Hα) scale as ∝L bH α0.7 for 1040 <1045 erg s-1. Following our earlier result that LbHα∝Lbol, this trend indicates that the relative narrow line luminosity decreases with increasing Lbol. 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 Lbol. Deviations from the above trend occur in objects with L/LEdd ≲ 10-2.6, probably due to the transition to LINERs with suppressed [Oiii] emission, and in objects with MBH > 108.5M⊙, probably due to the dominance of radio-loud AGN, and associated enhanced [Oiii] emission. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.