Since radio galaxies and quasars are seen at roughly the same cosmic epoch and because radio maps of quasars look similar to luminous FR-II radio galaxies, it is not difficult to understand that radio galaxies and quasars could be the same kind of objects. This alternative hypothesis has been developed by Barthel (1989). Based on the assumption that quasars were randomly orientated sources he couldn't explain certain observations like the fact that quasars are more bent at higher redshift (see figure 1). The only explanation he could think of responsible for this effect was a more dense and clumpier intergalactic medium at high redshift. But this should also have the same effect on radio galaxies, which is not observed. This is one of the reasons why Barthel said that the assumption that quasars are orientated in a random fashion could not be true. He suggested that quasars are powerful double-lobed radio galaxies orientated preferentially close to the line of sight.
Due to relativistic effects, like Doppler boosting, there are some observational consequences like superluminal motion in the nuclei of quasars and active galaxies. The relativistic beaming model interprets the faster-than-light velocities (see chapter 3 for the mathematics involved) in terms of motion of highly relativistic plasma along an angle close to the line of sight.
A remarkable characteristic of the polarized radio emission of quasars is the so-called depolarization asymmetry (Garrington et al. 1988, Laing 1988) between the jetted and unjetted side of the source. If powerful radio galaxies have radio axes at larger inclination angles than quasars, then the depolarization asymmetry, which is very pronounced for quasars, is expected to be less strong for radio galaxies.
A problem that arises with this attempt to unify quasars and radio galaxies is that quasars usually have far stronger optical broad emission lines than radio galaxies. Since line emission is unlikely to be affected by Doppler boosting, this difference cannot be attributed to beaming effects. Barthel proposes that these observed differences can be explained by the presence of a torus of dust and gas around the radio axis, surrounding the broad line region. In powerful radio galaxies, the torus would be seen almost edge-on, and therefore hiding the broad line region and most of the nuclear continuum emission from the observer.
Another objection against the unification hypothesis is that the X-ray luminosity of quasars is much higher than that of radio galaxies. This cannot be explained by anisotropic absorption. It has also been suggested that there are intrinsic differences between the host galaxies and galactic environments of quasars and radio galaxies, but right now there is no conclusive evidence.
Unfortunately, many observations which are consistent with the idea that powerful radio galaxies are quasars inclined at large angles to the line of sight, can also be interpreted as evidence for the view that quasars are intrinsically more powerful objects than radio galaxies, or that quasars are radio galaxies undergoing a period of enhanced activity. The fact that many of the tests (bending angle versus redshift, angular size asymmetry, flux asymmetry, Laing-Garrington effect (see next paragraph)) performed by several authors are consistent with the quasar-radio galaxy unification model, is therefore no proof that the model is correct.
Figure 2: Schematic model of unification hypothesis