2003ApJ...590...86L

There is clear observational evidence that some narrow-line (type 2) active galactic nuclei (AGNs) have a hidden broad-line region (BLR) and are thus intrinsically broad-line (type 1) AGNs. Does this AGN unification apply for all type 2 AGNs? Indirect arguments suggest that some ``true'' type 2 AGNs, i.e., AGNs having no obscured BLR, do exist, but it is not clear why the BLR is missing in these AGNs. Here we point out a possible natural explanation. The observed radius-luminosity relation for the BLR implies an increasing line width with decreasing luminosity for a given black hole mass (M<sub>BH</sub>). In addition, there appears to be an upper limit to the observed width of broad emission lines in AGNs of Δv<sub>max</sub>∼25,000 km.s<sup>–1</sup>, which may reflect a physical limit above which the BLR may not be able to survive. Thus, at a low enough luminosity the BLR radius shrinks below the Δv<sub>max</sub> radius, leaving no region where the BLR can exist, although the AGN may remain otherwise ``normal.'' The implied minimum bolometric luminosity required to sustain a BLR with Δv<25,000 km.s^–1 is L_min∼10^41.8(M_BH/10⁸M_☉)². All AGNs with L<L_minare expected to be ``true'' type 2 AGNs, i.e., narrow-line AGNs without a hidden BLR. Predictions for the true nature of low-luminosity AGNs in two samples of nearby galaxies are provided. These can be used to test the above L_minconjecture and the predictions of other models for the size and origin of the BLR.