7.2. Internal and External Dust
The dust in external galaxies may not have exactly the same extinction properties as in our interstellar medium. In particular, some of the broad extinction features, such as the one centered at 2200Å, may be weak or absent in other galaxies. The amount of extinction will be estimated assuming a galactic dust-to-gas ratio and a simple, -1 extinction law. Thus, for a dust on the line of sight to the source, the extinction in magnitude, for a gas column density Ncol, is
Dust can also be mixed in with the gas, absorbing both the external incident radiation and the internally produced line photons. The first and large effect on the emergent spectrum is the extinction of the ionizing radiation in a wavelength dependent way. This can be incorporated into the photoionization calculations provided the extinction properties of the dust at 912Å are known.
Internal dust can also destroy line photons with an efficiency that depends on the wavelength and the optical depth of the line in question. For forbidden lines, intercombination lines, and all other lines of negligible optical depth, the absorption probability is simply [1 - exp(-dust)] and depends only on the line frequency. This is not the case for resonance lines and other lines of considerable optical depth, where the lengthening of the path before escape is considerable (about a factor of 5, see section 4.4.2) due to the large number of scatterings. Such line photons are easily destroyed by dust and the result is a considerable weakening of the large optical depth lines compared with all other lines. AGN observations do not show any large reduction in the strength of L, CIV1549, and other optically thick lines, compared with the calculated intensity of the intercombination lines like CIII]1909. Therefore, the amount of internal dust, at least in the BLR clouds, cannot be large.
It is easy to incorporate these effects into the calculations using the formalism described in chapter 4 (equations 42-46). The main complication is the unknown dust distribution, which may not be uniform. In particular, the neutral gas zone is a more likely location for the dust particle to survive the intense radiation of the central source. Finally, internal dust can also change the hydrogen line spectrum in a low density gas, by providing a de-excitation mechanism for some high energy levels, decreasing, in this way, the effective optical depth of the Lyman lines.