Editing Active galactic nucleus (section)

=== 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.