The Eddington Ratios in Seyfert 2 Galaxies with and without Hidden Broad-Line Regions

Using a large sample of 90 Seyfert 2 galaxies (Sy2s) with spectropolarimetric observations, we test the suggestion that the presence of hidden broad-line regions (HBLRs) in Sy2s depends upon the Eddington ratio. The stellar velocity dispersion and extinction-corrected [O III] luminosity are used to derive the masses of central supermassive black holes and the Eddington ratio. We find that (1) below an Eddington ratio threshold of 10-1.37, all of the objects but one are non-HBLR Sy2s, while at higher Eddington ratios there is no obvious distinction in the Eddington ratio and black hole mass distributions between Sy2s with and without HBLRs; (2) almost no low-luminosity Sy2s (e.g., L[O III] < 1041 ergs s-1) show HBLRs, regardless of the column density of neutral hydrogen; (3) for high luminosities, the possibility of detecting HBLR Sy2s is almost the same as that for non-HBLR Sy2s; and (4) when considering only Compton-thin Sy2s with higher [O III] luminosity (>1041 ergs s-1), the detectability of HBLRs is very high, ~85%. These results suggest that AGN luminosity plays a major role in the failure to detect HBLRs in low-luminosity Sy2s, while for high-luminosity Sy2s the detectability of HBLRs depends not only upon the AGN activity, but also upon the torus obscuration.