The first and obvious consequence of the spectral observations is the division into broad and narrow emission lines that seem to come from two distinct parts of the nucleus. The first is named the "Broad Line Region" (hereafter BLR). Its typical size, as deduced from the broad line variability, is 10-100 light-days Seyfert 1 galaxies, and up to a few light years in bright quasars. The electron density in the BLR is at least 108 cm-3, as judged from the absence of strong, broad forbidden lines, and the typical gas velocity is 3000-10,000 km s-1. The second region is the "Narrow Line Region" (hereafter NLR). Here the typical density is 103-106 cm-3, and the gas velocity 300-1000 km s-1. The NLR must be much larger than the BLR, since no clear variation of the narrow emission lines is observed in objects undergoing large continuum variations. The NLR is resolved by ground-based observations in several nearby Seyferts, showing dimensions of 100-300 pc. There are good theoretical reasons to believe that the NLR in bright quasars, in those cases where it is observed, is much larger than that, perhaps a few kpc in diameter.
The crude division into BLR and NLR works very well when modeling AGNs. In particular, there is no clear evidence, so far, for a transition region between the two, with intermediate dimension, density and velocity.
The general picture adopted here, and in many review papers on the subject, is that of a small continuum source, around a massive black hole, surrounded by a much larger emission line region. Support for the small dimension of the continuum source comes from the short time scale variation of the optical, ultraviolet and X-ray continuum. The situation may be different at longer wavelengths, where the observed continuum radiation can originate in a region comparable in size to the BLR. Most of the discussion in the following chapters is independent of this extra complication related to the dimension of the millimeter-infrared continuum source. The parts that are likely to depend on it are specifically explained.