7.3. Dust and Reddening in the NLR
The reddening of the narrow emission lines can easily be measured from the observed intensity of the Balmer hydrogen lines. The only modification that is required is a small correction to the predicted H / H, since the intrinsic narrow-line ratio in Seyfert 1 and Seyfert 2 galaxies is about 3.1, rather than the 2.8 calculated from simple recombination theory (possibly a small collisional contribution to H). Other potential reddening indicators for the NLR are the HeII lines at 4686, 3204 and 1640Å the [SII] lines at 10320, 6716+6731 and 4069+4076Å, and perhaps some [OII] lines. The HeII lines are the most promising in this respect, since the theoretical ratios are well known. However, so far only the hydrogen lines have been used, because of the low sensitivity of the pre HST ultraviolet experiments. A further observational limitation in Seyfert 1s and quasars is the uncertainty associated with the separation of the broad and narrow line profiles.
Observations of many narrow line AGNs clearly show a significant amount of line reddening. The amount range from very small to extremely large. In particular, some of the NLXGs show indications for a very large amount of line and continuum extinction. Some papers suggest that the amount of extinction is correlated with the inclination of the host galaxy, being larger in edge-on galaxies. There are other reasons to believe that the obscuring gas has a flattened distribution, thus the amount of extinction can vary a lot from object to object.
The narrow emission line profiles provide another indication for dust in NLRs. Many lines have a noticeable blue asymmetry which is interpreted as a combination of radial motion and dust obscuration. Line profiles are further discussed in chapter 9.