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 H alpha selected active galactic nuclei (AGN), defined in Paper I. We find a decrease in the narrow to broad H alpha luminosity (L-bH alpha) ratio with increasing L-bH alpha, such that both L([O III ] lambda 5007) and L(narrow H alpha) scale as proportional to L-bH alpha(0.7) for 10(40) < L-bH alpha < 10(45) erg s(-1). Following our earlier result that L-bH alpha proportional to L-bol, this trend indicates that the relative narrow line luminosity decreases with increasing Lbol. We derive L-bol/10(43) 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 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 less than or similar to 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-circle dot, probably due to the dominance of radio-loud AGN, and associated enhanced [O III] emission.