Spectral differences between radio galaxies and quasars

IT has been suggested recently1–4 that two classes of radio-loud extragalactic object, narrow-line radio galaxies and quasars, are intrinsically similar, differing only in the angle from which they are seen by the observer. In the simplest version of such models, one would expect that the [O III] 5,007-A line luminosities of the two types of object would be similar for a given extended radio luminosity and redshift. In spectroscopic observations of a sample of radio galaxies and quasars selected so as to be otherwise comparable, we find that this is not the case: the quasar [O III] luminosities exceed those of radio galaxies by factors of 5–10. Extinction by at least two magnitudes as a result of orientation differences is necessary for these observations to remain consistent with the radio galaxy/quasar unification hypothesis; we suggest that evidence for such extinction may exist in the form of high Hα/Hβ ratios in radio galaxies5–8.