4.3. Emission line spectra of radio galaxies
4.3.1. FR II radio galaxies
The average emission line properties of BLRGs and radio loud QSOs on the one hand and Seyfert 1s and radio quiet QSOs on the other hand appear to be very similar (; ; ). A bright unresolved point source always dominates the optical core of BLRGs (). Differences between the optical spectra of NLRGs and Seyfert 2s are very minor (). Most NLRGs show prominent broad Balmer lines in polarized light ().
However, a number of FR IIs are known with emission lines of very low excitation or lacking emission lines completely, being spectroscopically similar to FR Is; these objects have relatively low radio luminosities; they have been called low-excitation radio galaxies or LERGs (; ). A few of these objects have radio morphologies intermediate between the FR I and FR II classes; but most are bona fide FR IIs. The optical properties of LERGs are probably isotropic as no broad lines are detected although a few objects show strong radio cores and other indications of beaming; i.e. they should not contain a dust torus and should not show broad emission lines whatever their angle to the line of sight ().
Radio powers and narrow emission line luminosities are strongly correlated over five orders of magnitudes for high-excitation FR II galaxies (; ; ).