On the Nature of Low‐Luminosity Narrow‐Line Active Galactic Nuclei

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 BH ). In addition, there appears to be an upper limit to the observed width of broad emission lines in AGNs of Δ v max ~ 25,000 km s -1 , 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 Δ v max 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 Δ v < 25,000 km s -1 is L min ~ 10 41.8 ( M BH /10 8 M ☉ ) 2 . All AGNs with L < L min are 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 min conjecture and the predictions of other models for the size and origin of the BLR.