Editing Active galactic nucleus (section)

=== Radio-loud unification ===
Historically, work on radio-loud unification has concentrated upon high-luminosity radio-loud quasars. These can be unified with narrow-line radio galaxies in a manner that is directly analogous to the Seyfert 1/2 unification (but without the complication of much in the way of a reflection component: narrow-line radio galaxies show no nuclear optical continuum or reflected X-ray component, although they do occasionally show polarized broad-line emission). The large-scale radio structures of these objects provide compelling evidence that the orientation-based unified models really are true.<ref>{{Cite journal
| volume = 331
| issue = 6152
| pages = 149–151
| last = Laing
| first = R. A.
| title = The sidedness of jets and depolarization in powerful extragalactic radio sources
| journal = Nature
| date = 1988
| doi = 10.1038/331149a0
| bibcode = 1988Natur.331..149L | s2cid = 45906162
}}</ref><ref>{{Cite journal |last1=Garrington |first1=S. T. |last2=Leahy |first2=J. P. |last3=Conway |first3=R. G. |last4=Laing |first4=R. A. |date=1988 |title=A systematic asymmetry in the polarization properties of double radio sources with one jet |journal=Nature |volume=331 |issue=6152 |pages=147–149 |bibcode=1988Natur.331..147G |doi=10.1038/331147a0 |s2cid=4347023}}</ref><ref>{{Cite journal
| volume = 336
| pages = 606–611
| last = Barthel
| first = P. D.
| title = Is every quasar beamed?
| journal = Astrophysical Journal
| date = 1989
| doi = 10.1086/167038
| bibcode=1989ApJ...336..606B
}}</ref> X-ray evidence, where available, supports the unified picture: [[Radio galaxy|Radio galaxies]] show evidence of obscuration from a torus, while quasars do not, although care must be taken because radio-loud objects also have a soft unabsorbed jet-related component, and high resolution is necessary to separate-out thermal emission from the sources' large-scale hot-gas environment.<ref>{{Cite journal |last1=Belsole |first1=E. |last2=Worrall |first2=D. M. |last3=Hardcastle |first3=M. J. |date=2006 |title=High-redshift Faranoff-Riley type II radio galaxies: X-ray properties of the cores |journal=Monthly Notices of the Royal Astronomical Society |volume=366 |issue=1 |pages=339–352 |arxiv=astro-ph/0511606 |bibcode=2006MNRAS.366..339B |doi=10.1111/j.1365-2966.2005.09882.x |s2cid=9509179 |doi-access=free}}</ref> At very small angles to the line of sight, relativistic beaming dominates, and we see a [[blazar]] of some variety.

However, the population of radio galaxies is completely dominated by low-luminosity, low-excitation objects. These do not show strong nuclear emission lines, either broad or narrow, but rather, they have optical continua which appear to be entirely jet-related,<ref name="chiaberge" /> and their X-ray emission is also consistent with coming purely from a jet, with no heavily absorbed nuclear component in general.<ref name="hardcastle" /> These objects cannot be unified with quasars, even though they include some high-luminosity objects when looking at radio emission, since the torus can never hide the narrow-line region to the required extent, and because infrared studies show that they have no hidden nuclear component:<ref>{{Cite journal |last1=Ogle |first1=P. |last2=Whysong |first2=D. |last3=Antonucci |first3=R. |date=2006 |title=Spitzer Reveals Hidden Quasar Nuclei in Some Powerful FR II Radio Galaxies |journal=The Astrophysical Journal |volume=647 |issue=1 |pages=161–171 |arxiv=astro-ph/0601485 |bibcode=2006ApJ...647..161O |doi=10.1086/505337 |s2cid=15122568}}</ref> In fact, there is no evidence for a torus in these objects at all. Most likely, they form a separate class in which only jet-related emission is important. At small angles to the line of sight, they will appear as [[BL Lac objects]].<ref>{{Cite journal
| volume = 204
| pages = 23–27P
| last = Browne
| first = I. W. A.
| title = Is it possible to turn an elliptical radio galaxy into a BL Lac object?
| journal = Monthly Notices of the Royal Astronomical Society
| date = 1983
| bibcode = 1983MNRAS.204P..23B
| doi=10.1093/mnras/204.1.23p| doi-access = free
}}</ref>