The lack of broad-line regions in low accretion rate active galactic nuclei as evidence of their origin in the accretion disk

In this Letter, we present evidence suggesting that the absence or presence of hidden broad-line regions (HBLRs) in Seyfert 2 galaxies is regulated by the rate at which matter accretes onto a central supermassive black hole, in units of the Eddington rate. Evidence is based on data from a subsample of type 2 active galactic nuclei extracted from the Tran spectropolarimetric sample and made up of all those sources that also have good-quality X-ray spectra available and for which a bulge luminosity can be estimated. We use the intrinsic (i.e., unabsorbed) Xray luminosities of these sources and their black hole masses ( estimated by using the well-known relationship between nuclear mass and bulge luminosity in galaxies) to derive the nuclear accretion rate in Eddington units. We find that virtually all HBLR sources have accretion rates larger than a threshold value of (m) over dot(thres) similar or equal to 10(-3) (in Eddington units), while non-HBLR sources lie at (m) over dot less than or similar to (m) over dot(thres). These data nicely fit predictions from a model proposed by Nicastro in which the broad- line regions ( BLRs) are formed by accretion disk instabilities occurring in proximity of the critical radius at which the disk changes from gas pressure dominated to radiation pressure dominated. This radius diminishes with decreasing (m) over dot; for low enough accretion rates ( and therefore luminosities),. the critical radius becomes smaller than the innermost stable orbit and BLRs cannot form.