On The Nature of Low Luminosity Narrow Line AGN

There is clear observational evidence that some narrow line (type 2) AGN have a hidden broad line region (BLR), and are thus intrinsically broad line (type 1) AGN. Does this AGN unification applies for all type 2 AGN? Indirect arguments suggest that some "true" type 2 AGN, i.e. AGN having no obscured BLR do exist, but it is not clear why the BLR is missing in these AGN. 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 (Mbh). In addition, there appears to be an upper limit to the observed width of broad emission lines in AGN of Delta v_max~25,000 km/s, 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 Delta 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 Delta v<25,000 km/s is L_min~10^{41.8}(Mbh}/10^8M_sun)^2. All AGN with L<L_min are expected to be `true' type 2 AGN, i.e. narrow line AGN without a hidden BLR. Predictions for the true nature of low luminosity AGN 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.