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


E-mail: (JS); (AL)


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 math formula and math formula scale as math formula for math formula. Following our earlier result that LbHα ∝ Lbol, this trend indicates that the relative narrow line luminosity decreases with increasing Lbol. We derive math formula. This implies that the bolometric correction factor, math formula, decreases from 3000 at math formula to 300 at math formula. 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 [O iii] emission, and in objects with MBH > 108.5 M, probably due to the dominance of radio-loud AGN, and associated enhanced [O iii] emission.