The second model (Figure 36b) is similar to the first, but now the engine is producing a pair of jets that will eventually end in a pair of lobes, as seen in radio galaxies and some quasars.
Looking at the model from the side, one expects to see narrow lines in the spectrum (but not broad lines) and two jets surrounded by extended lobes. This is a narrow-line radio galaxy. At an angle closer to the jet axis the broad-line region comes into view and a broad-line radio galaxy is seen. So far this is analogous to the two types of Seyfert, but now another effect comes into play. As you saw in Section 4.7, relativistic beaming will cause an approaching jet to be brighter than a receding jet, so as the angle decreases one jet will fade at the expense of the other and a radio galaxy with a single jet will now be visible (though there may well be two lobes).
As the angle continues to decrease the intense source of radiation surrounding the black hole comes into view and the object appears as a quasar, with never more than one visible jet. Finally, a blazar is seen when the torus is face-on to the observer who is looking straight down the jet. One distinguishing feature of the blazars is that the spectrum is dominated by a smooth continuous spectrum which is what one would expect if the radiation is coming from the jet itself. Another feature of blazars is their rapid variability over a wide range of wavelengths, and this again is to consistent with the idea of the emission arising from a jet. BL Lacs would correspond to the less powerful radio galaxies and OVVs to the more powerful ones.
Unification of the radio-loud sources is more contentious and this model is by no means the last word on the subject. It has been difficult to reconcile all the observed properties of the AGNs with the model. For example, one test would be to examine whether the numbers of different kinds of AGN are consistent with what the model predicts.
Suppose that radio galaxies, radio-loud quasars and blazars were all the same kind of object but seen from different angles. From Figure 36b, which would you expect to be the most common? Which the least common?
Radio galaxies would be seen over the widest range of angles, so these would be the most common. Blazars, on the other hand, would only be seen over a narrow range of angles and would be relatively rare.
This simple approach is complicated by two things. First, AGNs vary greatly in luminosity and distance, so the number observed is not necessarily a measure of how common they are. Powerful or nearby objects are more likely to show up in a survey than weak or distant objects. Second, AGNs are visible over such large distances that the light from the more remote ones started on its journey when the Universe was considerably younger than it is today. The most distant quasars may no longer exist in the form in which they are observed. We shall return to that idea shortly.
At the moment the jury is still out, as they say, but astronomers are confident that even if the different kinds of radio-loud AGNs are not identical siblings, they are at least close cousins.
Perhaps the most difficult question is why some AGNs are radio-loud while most are radio-quiet. You have seen that the radio-quiet AGNs appear to reside in spiral galaxies while the radio-loud AGNs are in ellipticals. It was once thought that the presence of gas in spiral galaxies acted to suppress the emergence of jets from the engine, but that idea is no longer favoured. Current thinking relates the presence of jets to the angular momentum of the black hole, with only the faster-spinning black holes able to produce jets. The novel element is that a high spin rate could be achieved not by accretion but by the merger of two massive black holes following the collision and merger of their host galaxies. There is other evidence that giant elliptical galaxies are formed from mergers, so this seems a plausible, if yet unproven, explanation as to why the radio-loud sources tend to be found in ellipticals.