Editing Active galactic nucleus (section)

=== Radio-loud AGN ===
There are several subtypes of radio-loud active galactic nuclei.

* Radio-loud quasars behave exactly like radio-quiet quasars with the addition of emission from a jet. Thus they show strong optical continuum emission, broad and narrow emission lines, and strong X-ray emission, together with nuclear and often extended radio emission.
* "[[Blazar]]s" ([[BL Lac object|BL Lacertae (BL Lac) objects]] and [[OVV quasar|optically violent variable (OVV) quasars]]) are distinguished by rapidly variable, polarized optical, radio, and X-ray emissions. BL Lac objects show no optical emission lines, broad or narrow, so that their redshifts can only be determined from features in the spectra of their host galaxies. The emission-line features may be intrinsically absent, or simply swamped by the additional variable component. In the latter case, emission lines may become visible when the variable component is at a low level.<ref>{{Cite journal
| volume = 452
| issue = 1
| pages = 5–8
| last1 = Vermeulen
| first1 = R. C.
| last2 = Ogle
| first2 = P. M.
| title = When Is BL Lac Not a BL Lac?
| journal = The Astrophysical Journal Letters
| doi = 10.1086/309716
| date = 1995
| bibcode = 1995ApJ...452L...5V
| last3 = Tran
| first3 = H. D.
| last4 = Browne
| first4 = I. W. A.
| last5 = Cohen
| first5 = M. H.
| last6 = Readhead
| first6 = A. C. S.
| last7 = Taylor
| first7 = G. B.
| last8 = Goodrich
| first8 = R. W.
| doi-access = free
}}</ref> OVV quasars behave more like standard radio-loud quasars with the addition of a rapidly variable component. In both classes of source, the variable emission is believed to originate in a relativistic jet that is oriented close to the line of sight. Relativistic effects amplify both the luminosity of the jet and the amplitude of variability.
* [[radio galaxy|Radio galaxies]]. These objects show nuclear and extended radio emission. Their other AGN properties are heterogeneous. They can broadly be divided into low-excitation and high-excitation classes.<ref>{{Cite journal |last1=Hine |first1=R. G. |last2=Longair |first2=M. S. |date=1979 |title=Optical spectra of 3 CR radio galaxies |journal=Monthly Notices of the Royal Astronomical Society |volume=188 |pages=111–130 |bibcode=1979MNRAS.188..111H |doi=10.1093/mnras/188.1.111 |doi-access=free}}</ref><ref>{{Cite journal |last1=Laing |first1=R. A. |last2=Jenkins |first2=C. R. |last3=Wall |first3=J. V. |last4=Unger |first4=S. W. |date=1994 |title=Spectrophotometry of a Complete Sample of 3CR Radio Sources: Implications for Unified Models |journal=The First Stromlo Symposium: The Physics of Active Galaxies. ASP Conference Series |volume=54 |pages=201 |bibcode=1994ASPC...54..201L}}</ref> Low-excitation objects show no strong narrow or broad emission lines, and the emission lines they do have may be excited by a different mechanism.<ref>{{Cite journal
| volume = 451
| page = 88
| last1 = Baum
| first1 = S. A.
| first2 = E. L.
| last2 = Zirbel
| title = Toward Understanding the Fanaroff-Riley Dichotomy in Radio Source Morphology and Power
| journal = The Astrophysical Journal
| date = 1995
| doi = 10.1086/176202
| bibcode=1995ApJ...451...88B
| last3 = O'Dea
| first3 = Christopher P.
}}</ref> Their optical and X-ray nuclear emission is consistent with originating purely in a jet.<ref name="chiaberge">{{Cite journal |last1=Chiaberge |first1=M. |last2=Capetti |first2=A. |last3=Celotti |first3=A. |date=2002 |title=Understanding the nature of FRII optical nuclei: a new diagnostic plane for radio galaxies |journal=Astron. Astrophys. |volume=394 |issue=3 |pages=791–800 |arxiv=astro-ph/0207654 |bibcode=2002A&A...394..791C |doi=10.1051/0004-6361:20021204 |s2cid=4308057}}</ref><ref name="hardcastle">{{Cite journal |last1=Hardcastle |first1=M. J. |last2=Evans |first2=D. A. |last3=Croston |first3=J. H. |date=2006 |title=The X-ray nuclei of intermediate-redshift radio sources |journal=Monthly Notices of the Royal Astronomical Society |volume=370 |issue=4 |pages=1893–1904 |arxiv=astro-ph/0603090 |bibcode=2006MNRAS.370.1893H |doi=10.1111/j.1365-2966.2006.10615.x |s2cid=14632376 |doi-access=free}}</ref> They may be the best current candidates for AGN with radiatively inefficient accretion. By contrast, high-excitation objects (narrow-line radio galaxies) have emission-line spectra similar to those of Seyfert 2s. The small class of broad-line radio galaxies, which show relatively strong nuclear optical continuum emission<ref>{{Cite journal |last1=Grandi |first1=S. A. |last2=Osterbrock |first2=D. E. |date=1978 |title=Optical spectra of radio galaxies |journal=Astrophysical Journal |volume=220 |issue=Part 1 |page=783 |bibcode=1978ApJ...220..783G |doi=10.1086/155966}}</ref> probably includes some objects that are simply low-luminosity radio-loud quasars. The host galaxies of radio galaxies, whatever their emission-line type, are essentially always ellipticals.

<!-- Needs extending -->
{| class="wikitable" style="margin:auto;"
 |+ Features of different types of galaxies
 ! rowspan=2 | Galaxy type
 ! rowspan=2 | Active<br>nuclei
 ! colspan="2" |Emission lines
 ! rowspan=2 | X-rays
 ! colspan="2" |Excess of
 ! rowspan=2 |Strong<br>radio
 ! rowspan=2 |Jets
 ! rowspan=2 |Variable
 ! rowspan=2 |Radio<br>loud
 |-
 !Narrow !! Broad !! UV !! Far-IR
 |-
 !Normal (non-AGN)
 | no || weak || no || weak || no || no || no || no || no || no
 |-
 ![[LINER]]
 | unknown || weak || weak || weak || no || no || no || no || no || no
 |-
 ![[Seyfert galaxy|Seyfert]] I
 | yes || yes || yes || some || some || yes || few || no || yes || no
 |-
 !Seyfert II
 | yes || yes || no || some || some || yes || few || no || yes || no
 |-
 ![[Quasar]]
 | yes || yes || yes || some || yes || yes || some || some || yes || some
 |-
 ![[Blazar]]
 | yes || no || some || yes || yes || no ||yes || yes || yes ||yes
 |-
 ![[BL Lac object|BL Lac]]
 | yes || no || no/faint || yes || yes || no || yes || yes || yes || yes
 |-
 ![[OVV quasar|OVV]]
 | yes || no || stronger than BL Lac|| yes || yes || no || yes || yes || yes || yes
 |-
 ![[Radio galaxy]]
 | yes || some || some || some || some || yes || yes || yes || yes || yes
 |-
|}