Much of the previous analysis focused on the broad emission lines. This limits the discussion to the more luminous AGNs, where these lines are strong and easy to measure. The aim in this chapter is to concentrate on fainter AGNs, those in which the broad lines are weak or absent. These are mostly low redshift objects showing some, but not all the AGN characteristics. Many of their observed properties have already been discussed in chapters 1 and 2, and the emphasis here is on the physical interpretation. At the end of the chapter we use the observed narrow-line properties to suggest a classification scheme for the entire AGN family.
11.1 Narrow Line X-ray Galaxies
Many of these objects have been discovered in X-ray surveys and identified, subsequently, with emission line galaxies. Some of their observed characteristics have been discussed in chapter 1. They include strong narrow lines, mostly of high excitation, strong X-ray emission and weak ultraviolet continuum. There is a great similarity in the spectral distribution of the X-ray continuum of NLXGs and Seyfert 1 galaxies.
The key to the understanding of NLXGs is the faint, broad
H
observed
in many of them. In these objects the broad
H
is
extremely weak, or absent, giving a strong lower limit to the broad
H /
H
ratio of ~ 10. This can be
understood if NLXGs are heavily reddened Seyfert 1 galaxies. Most observed
differences between NLXGs and "normal" Seyfert 1s are consistent with this
idea. The amount of dust is large, AV ~ 2 mag. or so,
but not large enough to
absorb the hard X-ray radiation. This explains the weak ultraviolet
continuum.
There are some indications that the amount of reddening is correlated with
the inclination of the host galaxy, with edge-on spirals showing the largest
extinction. This suggests that reddening may be common in many AGNs, being
less noticeable in the so called "normal" Seyferts, which, in this
scheme, are
located in face-on host galaxies. The dust is associated, most probably,
with
the interstellar medium, and is not necessarily related to the dust
suspected to cause some of the broad line reddening, closer to the center
(chapter 7).