1.4 Narrow Line X-ray Galaxies (NLXGs)
The name NLXGs was invented to describe some narrow line galaxies with unusual X-ray properties. The more luminous objects of this class show strong narrow lines, similar in width and excitation to the narrow lines of Seyfert 1 and Seyfert 2 galaxies. The NLXGs are strong X-ray emitters, with optical-to-X-ray continuum luminosity ratio similar to Seyfert ls. The ultraviolet spectrum of most NLXGs is weaker than that of Seyfert 1s and there are clear indications (chapter 11) that this may be due to reddening. A typical optical spectrum is similar to the Seyfert 2 spectrum shown in Fig.1, except that in some cases there exists a weak, very broad wing to H. Some NLXGs show much lower excitation lines, and are more similar in this respect to LINERs. Thus, this group of objects is difficult to classify and there is a definite overlap in properties with the previously discussed groups.
Some powerful radio galaxies show a characteristic Seyfert 2 type spectrum. These have been named Narrow Line Radio Galaxies (NLRGs) and are sometimes classified as a separate subgroup of AGN. Here they are classified as either Seyfert 2 galaxies or NLXGs, depending on their X-ray luminosity.
Some AGNs do not show any emission lines. These are the BL Lac objects, identified by their highly variable nonthermal continuum. They are not discussed in this contribution, which is aimed at understanding AGNs by means of analyzing their emission lines. The properties of BL Lac objects are discussed by other authors in this book.
The above observational classification is useful, but may not reveal much of the nature of AGNs without a deeper understanding of the spectrum and the information that can be extracted from it. This is the subject of this contribution. A more meaningful classification scheme is proposed in chapter 11, after all these issues have been discussed.