ABSTRACT

The apparent low covering factor of the narrow line region (NLR), the nearly “empty” intermediate region between the broad line region (BLR) and the NLR, and the size of the BLR, are all naturally explained if dust is embedded in the narrow line emitting gas. Such dust, together with the observed blue-excess asymmetry of the narrow line profiles, imply that the NLR gas has a net inflow motion.

THE BASIC QUESTIONS

In this paper we address the following three questions: 1) Why does RBLR ∼ 0.1 L½46 pc? This scaling is indicated by the remarkably similar emission spectra of AGNs over a very large luminosity range. 2) What is the actual covering factor (C) of the cold gas in the NLR? Photon counting using the Hβ recombination line indicates C ∼ 2%, assuming optically thick clouds. The 3 – 30 μm IR continuum, if due to dust reprocessing of the UV continuum, indicates C ∼ 20 – 40%. High resolution imagings of the NLR by the HST in nearby AGNs indicate, if the clouds are resolved, C ≥ 20%. 3). Why is there an apparent gap in the gas distribution between the BLR and the NLR? A gap is indicated most clearly in Seyfert 1.5 galaxies and some quasars where the permitted line profiles show distinct narrow and broad components.