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

There is clear observational evidence that some narrow line (type 2) AGN havea hidden broad line region (BLR), and are thus intrinsically broad line (type1) AGN. Does this AGN unification applies for all type 2 AGN? Indirectarguments suggest that some "true" type 2 AGN, i.e. AGN having no obscured BLRdo exist, but it is not clear why the BLR is missing in these AGN. Here wepoint out a possible natural explanation. The observed radius-luminosityrelation for the BLR implies an increasing line width with decreasingluminosity for a given black hole mass (Mbh). In addition, there appears to bean upper limit to the observed width of broad emission lines in AGN of Deltav_max~25,000 km/s, which may reflect a physical limit above which the BLR maynot be able to survive. Thus, at a low enough luminosity the BLR radius shrinksbelow the Delta v_max radius, leaving no region where the BLR can exist,although the AGN may remain otherwise `normal'. The implied minimum bolometricluminosity required to sustain a BLR with Delta v<25,000 km/s isL_min~10^{41.8}(Mbh}/10^8M_sun)^2. All AGN with L