Support for a unified model of radio galaxies and quasars from isotropic [O _II] emission

UNIFIED models^1-3 of radio-loud quasars and powerful radio galaxies suggest that they are intrinsically similar objects observed from different angles. This can be tested by comparing the isotropically emitted radiation from the spatially extended nebulae surrounding the nuclei; the unified models predict that the intensities of these emissions should be comparable for the two classes of object. But when this prediction was examined⁴ for the [O _III] 5,007-Å emission line, it was found that quasar [O _III] luminosities significantly exceed those of otherwise similar radio galaxies. We have measured the spatially integrated [O _II] 3,727-Å emission-line luminosities for a number of quasars and radio galaxies taken from the 3C catalogue. Supplementing our data with values from the literature, we find no systematic difference in the [O _II] luminosities. We argue that this emission is indeed isotropic, and that our results are consistent with the unification hypothesis; the [O _III] line, on the other hand, may still have a significant component from the nuclear region, and thus be subject to pronounced anisotropic obscuration.