Editing Active galactic nucleus (section)

== Unification of AGN species ==
[[File:Emmaalexander unified agn.png|thumb|upright=1.6|Unified AGN models]]
Unified models propose that different observational classes of AGN are a single type of physical object observed under different conditions. The currently favoured unified models are 'orientation-based unified models' meaning that they propose that the apparent differences between different types of objects arise simply because of their different orientations to the observer.<ref>{{Cite journal
| volume = 31
| issue = 1
| pages = 473–521
| last = Antonucci
| first = R.
| title = Unified Models for Active Galactic Nuclei and Quasars
| journal = Annual Review of Astronomy and Astrophysics
| date = 1993
| doi = 10.1146/annurev.aa.31.090193.002353
| bibcode=1993ARA&A..31..473A
}}</ref><ref>{{Cite journal |last1=Urry |first1=P. |last2=Padovani |first2=Paolo |date=1995 |title=Unified schemes for radioloud AGN |journal=Publications of the Astronomical Society of the Pacific |volume=107 |pages=803–845 |arxiv=astro-ph/9506063 |bibcode=1995PASP..107..803U |doi=10.1086/133630 |s2cid=17198955}}</ref> However, they are debated (see below).

=== Radio-quiet unification ===
At low luminosities, the objects to be unified are [[Seyfert galaxy|Seyfert galaxies]]. The unification models propose that in Seyfert 1s the observer has a direct view of the active nucleus. In Seyfert 2s the nucleus is observed through an obscuring structure which prevents a direct view of the optical continuum, broad-line region or (soft) [[X-ray]] emission. The key insight of orientation-dependent accretion models is that the two types of object can be the same if only certain angles to the line of sight are observed. The standard picture is of a [[torus]] of obscuring material surrounding the accretion disc. It must be large enough to obscure the broad-line region but not large enough to obscure the narrow-line region, which is seen in both classes of object. Seyfert 2s are seen through the torus. Outside the torus there is material that can scatter some of the nuclear emission into our line of sight, allowing us to see some optical and X-ray continuum and, in some cases, broad emission lines—which are strongly polarized, showing that they have been scattered and proving that some Seyfert 2s really do contain hidden Seyfert 1s. Infrared observations of the nuclei of Seyfert 2s also support this picture.

At higher luminosities, quasars take the place of Seyfert 1s, but, as already mentioned, the corresponding 'quasar 2s' are elusive at present. If they do not have the scattering component of Seyfert 2s they would be hard to detect except through their luminous narrow-line and hard X-ray emission.

=== 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>

=== Criticism of the radio-quiet unification ===
In the recent literature on AGN, being subject to an intense debate, an increasing set of observations appear to be in conflict with some of the key predictions of the Unified Model, e.g. that each Seyfert 2 has an obscured Seyfert 1 nucleus (a hidden broad-line region).

Therefore, one cannot know whether the gas in all Seyfert 2 galaxies is ionized due to photoionization from a single, non-stellar continuum source in the center or due to shock-ionization from e.g. intense, nuclear starbursts. Spectropolarimetric studies<ref>{{Cite journal |last=Tran |first=H. D. |date=2001 |title=Hidden Broad-Line Seyfert 2 Galaxies in the CFA and 12 $\mu$M Samples |journal=The Astrophysical Journal |volume=554 |issue=1 |pages=L19–L23 |arxiv=astro-ph/0105462 |bibcode=2001ApJ...554L..19T |doi=10.1086/320926 |s2cid=2753150}}</ref> reveal that only 50% of Seyfert 2s show a hidden broad-line region and thus split Seyfert 2 galaxies into two populations. The two classes of populations appear to differ by their luminosity, where the Seyfert 2s without a hidden broad-line region are generally less luminous.<ref>{{Cite journal
| volume =730
| issue =2
| pages =121–130
| last1 = Wu
| first1 =Y-Z
| title = The Different Nature in Seyfert 2 Galaxies With and Without Hidden Broad-line Regions
| journal = The Astrophysical Journal
| date = 2001
| doi = 10.1088/0004-637X/730/2/121
| arxiv = 1101.4132 |bibcode = 2011ApJ...730..121W |last2=Zhang|first2=En-Peng| last3 =Liang
| first3 =Yan-Chun
| last4 =Zhang
| first4 =Cheng-Min
| last5 =Zhao
| first5 =Yong-Heng
| s2cid =119209693
| display-authors=1}}</ref> This suggests absence of broad-line region is connected to low Eddington ratio, and not to obscuration.

The covering factor of the torus might play an important role. Some torus models<ref>{{Cite journal
| volume =648
| issue = 2
| pages = L101–L104
| last = Elitzur
| first =M.
| author2=Shlosman I.
| title = The AGN-obscuring Torus: The End of the Doughnut Paradigm?
| journal = The Astrophysical Journal
| date = 2006
| doi = 10.1086/508158
| arxiv = astro-ph/0605686|bibcode = 2006ApJ...648L.101E | s2cid = 1972144
}}</ref><ref>{{Cite journal
| volume =530
| issue =2
| pages =L101–L104
| last = Nicastro
| first =F.
| title = Broad Emission Line Regions in Active Galactic Nuclei: The Link with the Accretion Power
| journal = The Astrophysical Journal
| date = 2000
| doi = 10.1086/312491
| pmid =10655166
| arxiv = astro-ph/9912524|bibcode = 2000ApJ...530L..65N | s2cid =23313718
}}</ref> predict how Seyfert 1s and Seyfert 2s can obtain different covering factors from a luminosity and accretion rate dependence of the torus covering factor, something supported by studies in the x-ray of AGN.<ref>{{Cite journal |last1=Ricci |first1=C. |last2=Walter |first2=R. |last3=Courvoisier |first3=T. J-L. |last4=Paltani |first4=S. |date=2010 |title=Reflection in Seyfert galaxies and the unified model of AGN |journal=Astronomy and Astrophysics |volume=532 |pages=A102–21 |arxiv=1101.4132 |bibcode=2011A&A...532A.102R |doi=10.1051/0004-6361/201016409 |s2cid=119309875}}</ref> The models also suggest an accretion-rate dependence of the broad-line region and provide a natural evolution from more active engines in Seyfert 1s to more "dead" Seyfert 2s<ref>{{Cite journal |last1=Wang |first1=J. M. |last2=Du |first2=P. |last3=Baldwin |first3=J. A. |last4=Ge |first4=J-Q. |last5=Ferland |first5=G. J. |last6=Ferland |first6=Gary J. |date=2012 |title=Star formation in self-gravitating disks in active galactic nuclei. II. Episodic formation of broad-line regions |journal=The Astrophysical Journal |volume=746 |issue=2 |pages=137–165 |arxiv=1202.0062 |bibcode=2012ApJ...746..137W |doi=10.1088/0004-637X/746/2/137 |s2cid=5037595}}</ref> and can explain the observed break-down of the unified model at low luminosities<ref>{{Cite journal
| volume =590
| issue =1
| pages =86–94
| last = Laor
| first =A.
| title = On the Nature of Low-Luminosity Narrow-Line Active Galactic Nuclei
| journal = The Astrophysical Journal
| date = 2003
| doi = 10.1086/375008|arxiv = astro-ph/0302541 |bibcode = 2003ApJ...590...86L | s2cid =118648122
}}</ref> and the evolution of the broad-line region.<ref>{{Cite journal |last1=Elitzur |first1=M. |last2=Ho |first2=L. C. |last3=Trump |first3=J. R. |date=2014 |title=Evolution of broad-line emission from active galactic nuclei |journal=Monthly Notices of the Royal Astronomical Society |volume=438 |issue=4 |pages=3340–3351 |arxiv=1312.4922 |bibcode=2014MNRAS.438.3340E |doi=10.1093/mnras/stt2445 |s2cid=52024863 |doi-access=free}}</ref>

While studies of single AGN show important deviations from the expectations of the unified model, results from statistical tests have been contradictory. The most important short-coming of statistical tests by direct comparisons of statistical samples of Seyfert 1s and Seyfert 2s is the introduction of selection biases due to anisotropic selection criteria.<ref>{{Cite journal
| volume =747
| issue =2
| pages =L33–L35
| last = Elitzur
| first =M.
| title = On the Unification of Active Galactic Nuclei
| journal = Astrophysical Journal Letters
| date = 2012
| doi = 10.1088/2041-8205/747/2/L33
| arxiv = 1202.1776 |bibcode = 2012ApJ...747L..33E | s2cid =5037009
}}</ref><ref>{{Cite journal
| last = Antonucci
| first =R.
| title = A panchromatic review of thermal and nonthermal active galactic nuclei
| journal =Astronomical and Astrophysical Transactions
| volume =27
| issue =4
| pages =557
| arxiv =1210.2716| year =2012
| bibcode =2012A&AT...27..557A
}}</ref>

Studying neighbour galaxies rather than the AGN themselves<ref>{{Cite journal
| volume =293
| pages = 683
| last = Laurikainen
| first = E.
| author2=Salo H.
| title = Environments of Seyfert galaxies. II. Statistical analyses
| journal = Astronomy and Astrophysics
| date = 1995
| bibcode=1995A&A...293..683L}}</ref><ref>{{Cite journal |last1=Dultzin-Hacyan |first1=D. |author-link1=Deborah Dultzin |last2=Krongold |first2=Y. |last3=Fuentes-Guridi |first3=I. |last4=Marziani |first4=P. |date=1999 |title=The Close Environment of Seyfert Galaxies and Its Implication for Unification Models |journal=Astrophysical Journal Letters |volume=513 |issue=2 |pages=L111–L114 |arxiv=astro-ph/9901227 |bibcode=1999ApJ...513L.111D |doi=10.1086/311925 |s2cid=15568552}}</ref><ref>{{Cite journal
| volume =639
| issue =1
| pages =37–45
| last = Koulouridis
| first =E.
| author2=Plionis M.|author3=Chavushyan V.|author4=Dultzin-Hacyan D.|author4-link=Deborah Dultzin|author5=Krongold Y.|author6=Goudis C.
| title = Local and Large-Scale Environment of Seyfert Galaxies
| journal = Astrophysical Journal
| date = 2006
| doi = 10.1086/498421
| arxiv =astro-ph/0509843|bibcode = 2006ApJ...639...37K| s2cid =118938514
}}</ref> first suggested the numbers of neighbours were larger for Seyfert 2s than for Seyfert 1s, in contradiction with the Unified Model. Today, having overcome the previous limitations of small sample sizes and anisotropic selection, studies of neighbours of hundreds to thousands of AGN<ref>{{Cite journal |last1=Villarroel |first1=B. |last2=Korn |first2=A. J. |date=2014 |title=The different neighbours around Type-1 and Type-2 active galactic nuclei |journal=Nature Physics |volume=10 |issue=6 |pages=417–420 |arxiv=1211.0528 |bibcode=2014NatPh..10..417V |doi=10.1038/nphys2951 |s2cid=119199124}}</ref> have shown that the neighbours of Seyfert 2s are intrinsically dustier and more star-forming than Seyfert 1s and a connection between AGN type, host galaxy morphology and collision history. Moreover, angular clustering studies<ref>{{Cite journal |last1=Donoso |first1=E. |last2=Yan |first2=L. |last3=Stern |first3=D. |last4=Assef |first4=R. J. |date=2014 |title=The Angular Clustering of WISE-Selected AGN: Different Haloes for Obscured and Unobscured AGN |journal=The Astrophysical Journal |volume=789 |issue=1 |pages=44 |arxiv=1309.2277 |bibcode=2014ApJ...789...44D |doi=10.1088/0004-637X/789/1/44 |s2cid=118512526}}</ref> of the two AGN types confirm that they reside in different environments and show that they reside within dark matter halos of different masses. The AGN environment studies are in line with evolution-based unification models<ref>{{Cite journal |last1=Krongold |first1=Y. |last2=Dultzin-Hacyan |first2=D. |author2-link=Deborah Dultzin |last3=Marziani |first3=D. |date=2002 |title=The Circumgalactic Environment of Bright IRAS Galaxies |journal=Astrophysical Journal |volume=572 |issue=1 |pages=169–177 |arxiv=astro-ph/0202412 |bibcode=2002ApJ...572..169K |doi=10.1086/340299 |s2cid=17282005}}</ref> where Seyfert 2s transform into Seyfert 1s during merger, supporting earlier models of merger-driven activation of Seyfert 1 nuclei.

While controversy about the soundness of each individual study still prevails, they all agree on that the simplest viewing-angle based models of AGN Unification are incomplete. Seyfert-1 and Seyfert-2 seem to differ in star formation and AGN engine power.<ref>{{Cite journal |last1=Villarroel |first1=B. |last2=Nyholm |first2=A. |last3=Karlsson |first3=T. |last4=Comeron |first4=S. |last5=Korn |first5=A. |last6=Sollerman |first6=J. |last7=Zackrisson |first7=E. |date=2017 |title=AGN luminosity and stellar age – two missing ingredients for AGN unification as seen with iPTF supernovae |journal=The Astrophysical Journal |volume=837 |issue=2 |pages=110 |arxiv=1701.08647 |bibcode=2017ApJ...837..110V |doi=10.3847/1538-4357/aa5d5a |s2cid=67809219 |doi-access=free}}</ref>

While it still might be valid that an obscured Seyfert 1 can appear as a Seyfert 2, not all Seyfert 2s must host an obscured Seyfert 1. Understanding whether it is the same engine driving all Seyfert 2s, the connection to radio-loud AGN, the mechanisms of the variability of some AGN that vary between the two types at very short time scales, and the connection of the AGN type to small and large-scale environment remain important issues to incorporate into any unified model of active galactic nuclei.

A study of Swift/BAT AGN published in July 2022<ref name="Ananna 2022">{{cite journal | last1=Ananna | first1=Tonima Tasnim | last2=Weigel | first2=Anna K. | last3=Trakhtenbrot | first3=Benny | last4=Koss | first4=Michael J. | last5=Urry | first5=C. Megan | last6=Ricci | first6=Claudio | last7=Hickox | first7=Ryan C. | last8=Treister | first8=Ezequiel | last9=Bauer | first9=Franz E. | last10=Ueda | first10=Yoshihiro | last11=Mushotzky | first11=Richard | last12=Ricci | first12=Federica | last13=Oh | first13=Kyuseok | last14=Mejía-Restrepo | first14=Julian E. | last15=Brok | first15=Jakob Den | last16=Stern | first16=Daniel | last17=Powell | first17=Meredith C. | last18=Caglar | first18=Turgay | last19=Ichikawa | first19=Kohei | last20=Wong | first20=O. Ivy | last21=Harrison | first21=Fiona A. | last22=Schawinski | first22=Kevin | title=BASS. XXX. Distribution Functions of DR2 Eddington Ratios, Black Hole Masses, and X-Ray Luminosities | journal=The Astrophysical Journal Supplement Series | publisher=American Astronomical Society | volume=261 | issue=1 | date=2022-07-01 | issn=0067-0049 | doi=10.3847/1538-4365/ac5b64 | page=9| arxiv=2201.05603 | bibcode=2022ApJS..261....9A | s2cid=245986416 | doi-access=free }}</ref> adds support to the "radiation-regulated unification model" outlined in 2017.<ref name="Ricci 2017">{{cite journal | last1=Ricci | first1=Claudio | last2=Trakhtenbrot | first2=Benny | last3=Koss | first3=Michael J. | last4=Ueda | first4=Yoshihiro | last5=Schawinski | first5=Kevin | last6=Oh | first6=Kyuseok | last7=Lamperti | first7=Isabella | last8=Mushotzky | first8=Richard | last9=Treister | first9=Ezequiel | last10=Ho | first10=Luis C. | last11=Weigel | first11=Anna | last12=Bauer | first12=Franz E. | last13=Paltani | first13=Stephane | last14=Fabian | first14=Andrew C. | last15=Xie | first15=Yanxia | last16=Gehrels | first16=Neil | title=The close environments of accreting massive black holes are shaped by radiative feedback | journal=Nature | publisher=Springer Science and Business Media LLC | volume=549 | issue=7673 | year=2017 | issn=0028-0836 | doi=10.1038/nature23906 | pages=488–491| pmid=28959966 | arxiv=1709.09651 | bibcode=2017Natur.549..488R | s2cid=205260182 }}</ref> In this model, the relative accretion rate (termed the "Eddington ratio") of the black hole has a significant impact on the observed features of the AGN. [[Black hole|Black Holes]] with higher Eddington ratios appear to be more likely to be unobscured, having cleared away locally obscuring material in a very short timescale.