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Date and Time of the Query: 2019-08-18 T02:04:34 PDT
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For refcode 2003ApJ...590...86L:
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Copyright by American Astronomical Society. Reproduced by permission
2003ApJ...590...86L On the Nature of Low-Luminosity Narrow-Line Active Galactic Nuclei Laor, Ari Abstract. 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 {DELTA}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 {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^-1^ is L_min_~10^41.8^(M_BH_/10^8^M_sun_)^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. Keywords: Galaxies: Active, Galaxies: Nuclei, Galaxies: Quasars: General
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