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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 an SDSS based sample of broad H-alpha selected AGN, defined in paper I. We find a decrease in the narrow to broad H-alpha luminosity (L_bHa) ratio with increasing L_bHa, such that both L([OIII] lambda5007) and L(narrow H-alpha) scale as L_bHa^0.7 for 10^40 < L_bHa < 10^45 ergs s^-1. Following our earlier result that L_bHa \propto L_bol, this trend indicates that the relative narrow line luminosity decreases with increasing L_bol. We derive L_bol / 10^43 ergs s^-1 = 4000 (L([OIII]) / 10^43 ergs s^-1)^1.39. This implies that the bolometric correction factor, L_bol / L([OIII]), decreases from 3,000 at L_bol = 10^46.1 ergs s^-1 to 300 at L_bol = 10^42.5 ergs s^-1. At low luminosity, the narrow component dominates the observed H-alpha 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 > 10^8.5 M_Sun, probably due to the dominance of radio loud AGN, and associated enhanced [OIII] emission.