Editing Active galactic nucleus (section)

== Types of active galaxy ==
It is convenient to divide AGN into two classes, conventionally called radio-quiet and radio-loud. Radio-loud objects have emission contributions from both the jet(s) and the lobes that the jets inflate. These emission contributions dominate the luminosity of the AGN at radio wavelengths and possibly at some or all other wavelengths. Radio-quiet objects are simpler since jet and any jet-related emission can be neglected at all wavelengths.

AGN terminology is often confusing, since the distinctions between different types of AGN sometimes reflect historical differences in how the objects were discovered or initially classified, rather than real physical differences.

=== Radio-quiet AGN ===
* [[Low-ionization nuclear emission-line region]]s (LINERs). As the name suggests, these systems show only weak nuclear emission-line regions, and no other signatures of AGN emission. It is debatable<ref>{{cite journal |last1=Belfiore |first1=Francesco |title=SDSS IV MaNGA – spatially resolved diagnostic diagrams: a proof that many galaxies are LIERs |journal=Monthly Notices of the Royal Astronomical Society |date=September 2016 |volume=461 |issue=3 |page=3111  |doi=10.1093/mnras/stw1234 |doi-access=free |arxiv=1605.07189 |bibcode=2016MNRAS.461.3111B |s2cid=3353122 |url=https://academic.oup.com/mnras/article/461/3/3111/2608476}}</ref> whether all such systems are true AGN (powered by accretion on to a supermassive black hole). If they are, they constitute the lowest-luminosity class of radio-quiet AGN. Some may be radio-quiet analogues of the low-excitation radio galaxies (see below).
* [[Seyfert galaxies]]. Seyferts were the earliest distinct class of AGN to be identified. They show optical-range nuclear continuum emission, narrow and occasionally broad emission lines, occasionally strong nuclear X-ray emission, and sometimes a weak small-scale radio jet. Originally they were divided into two types, known as Seyfert 1 and Seyfert 2: Seyfert 1s show strong broad emission lines, while Seyfert 2s do not, and Seyfert 1s are more likely to show strong low-energy X-ray emission. Various forms of elaboration upon this scheme exist: For example, Seyfert 1s with relatively narrow broad lines are sometimes referred to as narrow-line Seyfert 1s. The host galaxies of Seyferts are usually spiral or irregular galaxies.
* Radio-quiet [[quasar]]s/QSOs. These are essentially more-luminous versions of Seyfert 1s. The distinction is arbitrary, and is usually expressed in terms of a limiting optical magnitude. Quasars were originally 'quasi-stellar' in optical images, because they had optical luminosities that were greater than that of their host galaxy. They always show strong optical continuum emission, X-ray continuum emission, and broad and narrow optical emission lines. Some astronomers use the term QSO (Quasi-Stellar Object) for this class of AGN, reserving 'quasar' for radio-loud objects, while other astronomers talk about radio-quiet and radio-loud quasars. The host galaxies of quasars can be spirals, irregulars, or ellipticals. There is a correlation between the quasar's luminosity and the mass of its host galaxy, in that the most luminous quasars inhabit the most massive galaxies (ellipticals).
* 'Quasar 2s'. By analogy with Seyfert 2s, these are objects with quasar-like luminosities, but without strong optical nuclear continuum emission or broad line emission. They are scarce in surveys, though a number of possible candidate quasar 2s have been identified.

=== 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
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| first6 = A. C. S.
| last7 = Taylor
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| 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
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| 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
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}}</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
 |-
|}