DUST IN THE NARROW-LINE REGION OF ACTIVE GALACTIC NUCLEI

Observations of active galactic nuclei (AGNs) show the presence of a cloud system with two well-separated regions of line emission, the narrow-line region (NLR) and the broad-line region (BLR). There are several unexplained features of the system. First, there is an apparent ''gap'' between the BLR and the NLR with almost no line emission. Second, the covering factor of the NLR clouds, as inferred from photoionization models, is an order of magnitude smaller than the covering factor of the BLR, but the observed infrared continuum and new high-resolution HST images suggest a much larger covering factor. Finally the BLR size scales with the source luminosity in a way which is not understood. In this Letter we show that the apparent low covering factor of the NLR, the nearly ''empty'' intermediate region, and the size of the BLR, are all naturally explained if dust is embedded in the narrow line-emitting gas. We present new photoionization calculations for a continuous distribution of clouds extending from the BLR to the NLR. Dust is present in almost all that region and absorbs most of the ionizing radiation incident on the NLR. The resulting line emission is heavily suppressed especially at small distances, where the gas is more highly ionized. The dust sublimes very close to the center, where the velocity is high, giving rise to strong, broad emission lines (the BLR). The sublimation radius matches the observed BLR size, and the region just outside of it where line emission is inefficient, is the apparent gap. We discuss other implications of dusty NLRs and several observational tests of this idea.