What Can We Learn from the Smallest AGN?

Quite a few things. In particular, reverberation mapping of NGC 4395, the lowest luminosity type 1 Active Galactic Nucleus (AGN, Lbol ∼ 10 erg/s) revealed a size of only ≃ 1 light hour for the C IV broad line region (BLR), which is by far the smallest BLR. This, together with a similar determination of a size of ∼ 200 light days in a luminous quasar (Kaspi et al. 2007), suggests that the RBLR ∝ L 1/2 relation holds over a range of 10 in L. This relation was suggested to result from dust sublimation, which sets RBLR. This suggestion was beautifully confirmed recently by the dust reverberation results of Suganuma et al. (2006). The RBLR ∝ L 1/2 relation implies that the broad lines width increases with decreasing luminosity according to ∆v ∝ L. But, there is an observational cutoff at ∆v ≃ 25, 000 km/s, and thus below a certain threshold L the BLR would not be detectable. Such objects constitute the socalled “true type 2” AGN (e.g. most FR I radio galaxies). The physical origin of the BLR gas is not established yet, but high quality Keck spectra of the Hα profile in NGC 4395 rule out a clumped distribution, and indicate that the gas resides in a smooth flow, most likely in a thick rotationally supported configuration. The Hα line also reveals extended exponential wings, which are well modeled by electron scattering within the BLR emitting gas. Such wings can be used as a direct probe of the BLR temperature and optical depth.