Editing Galaxy (section)

=== Active galaxy ===
{{Main|Active galactic nucleus}}
[[File:M87 jet.jpg|thumb|right|200px|A jet of particles is being emitted from the core of the elliptical radio galaxy [[Messier 87|M87]].]]

Some observable galaxies are classified as "active" if they contain an active galactic nucleus (AGN).{{sfn|Fraknoi et al.|2023|p=[https://openstax.org/books/astronomy-2e/pages/27-1-quasars 906]}} A significant portion of the galaxy's total energy output is emitted by the active nucleus instead of its stars, dust and [[interstellar medium]]. There are multiple classification and naming schemes for AGNs, but those in the lower ranges of luminosity are called [[Seyfert galaxy|Seyfert galaxies]], while those with luminosities much greater than that of the host galaxy are known as quasi-stellar objects or [[quasar]]s. Models of AGNs suggest that a significant fraction of their light is shifted to far-infrared frequencies because optical and UV emission in the nucleus is absorbed and remitted by dust and gas surrounding it.<ref>{{Cite journal |last1=Zakamska |first1=Nadia L. |last2=Strauss |first2=Michael A. |last3=Krolik |first3=Julian H. |last4=Collinge |first4=Matthew J. |last5=Hall |first5=Patrick B. |last6=Hao |first6=Lei |last7=Heckman |first7=Timothy M. |last8=Ivezi |first8=eljko |last9=Richards |first9=Gordon T. |last10=Schlegel |first10=David J. |last11=Schneider |first11=Donald P. |last12=Strateva |first12=Iskra |last13=Vanden Berk |first13=Daniel E. |last14=Anderson |first14=Scott F. |last15=Brinkmann |first15=Jon |date=November 2003 |title=Candidate Type II Quasars from the Sloan Digital Sky Survey. I. Selection and Optical Properties of a Sample at 0.3< Z  |url=https://iopscience.iop.org/article/10.1086/378610 |journal=[[The Astronomical Journal]] |language=en |volume=126 |issue=5 |pages=2125–2144 |doi=10.1086/378610 |issn=0004-6256|arxiv=astro-ph/0309551 }}</ref>

The standard model for an [[active galactic nucleus]] is based on an [[accretion disc]] that forms around a [[supermassive black hole]] (SMBH) at the galaxy's core region. The radiation from an active galactic nucleus results from the [[gravitational energy]] of matter as it falls toward the black hole from the disc.{{sfn|Fraknoi et al.|2023|pp=[https://openstax.org/books/astronomy-2e/pages/27-2-supermassive-black-holes-what-quasars-really-are 907–915]}}<ref name="keel">{{cite web
 |last1=Keel
 |first1=William C.
 |date=2000
 |url=http://www.astr.ua.edu/keel/galaxies/agnintro.html
 |title=Introducing Active Galactic Nuclei
 |publisher=[[University of Alabama]]
 |access-date=December 6, 2006
 |archive-date=April 27, 2012
 |archive-url=https://web.archive.org/web/20120427123358/http://www.astr.ua.edu/keel/galaxies/agnintro.html
 |url-status=live
 }}</ref> The AGN's luminosity depends on the SMBH's mass and the rate at which matter falls onto it.
In about 10% of these galaxies, a diametrically opposed pair of [[Astrophysical jet|energetic jets]] ejects particles from the galaxy core at velocities close to the [[speed of light]]. The mechanism for producing these jets is not well understood.<ref name="monster">{{cite web
 |last1=Lochner
 |first1=Jim
 |editor-last=Gibb
 |editor-first=Meredith
 |title=A Monster in the Middle
 |url=http://imagine.gsfc.nasa.gov/docs/science/know_l2/active_galaxies.html
 |website=Imagine the Universe!
 |publisher=[[NASA]]
 |access-date=December 20, 2006
 |archive-date=March 26, 2009
 |archive-url=https://web.archive.org/web/20090326220155/http://imagine.gsfc.nasa.gov/docs/science/know_l2/active_galaxies.html
 |url-status=dead
 }}</ref>

==== Seyfert galaxy ====
{{Main|Seyfert galaxy}}

Seyfert galaxies are one of the two largest groups of active galaxies, along with quasars. They have quasar-like nuclei (very luminous, distant and bright sources of electromagnetic radiation) with very high surface brightnesses; but unlike quasars, their host galaxies are clearly detectable.<ref name=Peterson1997>{{cite book |url=http://ned.ipac.caltech.edu/level5/Cambridge/frames.html |title=An Introduction to Active Galactic Nuclei |publisher=[[Cambridge University Press]] |first=Bradley M. |last=Peterson |year=1997 |isbn=978-0-521-47911-0}}</ref> Seen through a telescope, a Seyfert galaxy appears like an ordinary galaxy with a bright star superimposed atop the core. Seyfert galaxies are divided into two principal subtypes based on the frequencies observed in their spectra.<ref>{{Cite journal |last=Peterson |first=Bradley M. |date=2008-08-01 |title=The central black hole and relationships with the host galaxy |url=https://www.jmmc.fr/mirrors/www.vlti.org/events/assets/3/documents/peterson.pdf |journal=New Astronomy Reviews |series=Active Galactic Nuclei at the Highest Angular Resolution: Theory and Observations |volume=52 |issue=6 |pages=240–252 |bibcode=2008NewAR..52..240P |doi=10.1016/j.newar.2008.06.005 |s2cid=121460317 |issn=1387-6473}}</ref>

==== Quasar ====
{{Main|Quasar}}

Quasars are the most energetic and distant members of active galactic nuclei. Extremely luminous, they were first identified as high redshift sources of electromagnetic energy, including radio waves and visible light, that appeared more similar to stars than to extended sources similar to galaxies. Their luminosity can be 100 times that of the Milky Way.{{sfn|Fraknoi et al.|2023|pp=[https://openstax.org/books/astronomy-2e/pages/27-1-quasars 899–906]}} The nearest known quasar, [[Markarian 231]], is about 581 million light-years from Earth,<ref>{{Cite journal |last1=Kovačević |first1=Andjelka B |last2=Yi |first2=Tignfeng |last3=Dai |first3=Xinyu |last4=Yang |first4=Xing |last5=Čvorović-Hajdinjak |first5=Iva |last6=Popović |first6=Luka Č |date=2020-05-21 |title=Confirmed short periodic variability of subparsec supermassive binary black hole candidate Mrk 231 |url=https://academic.oup.com/mnras/article/494/3/4069/5809369 |journal=[[Monthly Notices of the Royal Astronomical Society]] |language=en |volume=494 |issue=3 |pages=4069–4076 |doi=10.1093/mnras/staa737 |doi-access=free |issn=0035-8711|arxiv=2003.06359 }}</ref> while others have been discovered as far away as [[UHZ1]], roughly 13.2 billion light-years distant.<ref>{{Cite web |date=2023-11-10 |title=UHZ1: Distant Galaxy and Black Hole |url=https://apod.nasa.gov/apod/ap231110.html |access-date=2024-02-18 |editor-last1=Nemiroff|editor-first1=Robert|editor-last2=Bonnell|editor-first2=Jerry|website=[[Astronomy Picture of the Day]]|publisher=[[NASA]], [[Michigan Technological University]]}}</ref><ref>{{Cite journal |last1=Bogdán |first1=Ákos |last2=Goulding |first2=Andy D. |last3=Natarajan |first3=Priyamvada |last4=Kovács |first4=Orsolya E. |last5=Tremblay |first5=Grant R. |last6=Chadayammuri |first6=Urmila |last7=Volonteri |first7=Marta |author7-link=Marta Volonteri|last8=Kraft |first8=Ralph P. |last9=Forman |first9=William R. |last10=Jones |first10=Christine |last11=Churazov |first11=Eugene |last12=Zhuravleva |first12=Irina |date=January 2024 |title=Evidence for heavy-seed origin of early supermassive black holes from a z ≈ 10 X-ray quasar |url=https://www.nature.com/articles/s41550-023-02111-9 |journal=[[Nature Astronomy]] |language=en |volume=8 |issue=1 |pages=126–133 |arxiv=2305.15458 |bibcode=2024NatAs...8..126B |doi=10.1038/s41550-023-02111-9 |s2cid=258887541 |issn=2397-3366}}</ref> Quasars are noteworthy for providing the first demonstration of the phenomenon that [[Gravitational lens|gravity can act as a lens for light]].<ref>{{Cite journal |last1=Blandford |first1=R. D. |last2=Narayan |first2=R. |date=September 1992 |title=Cosmological Applications of Gravitational Lensing |url=https://www.annualreviews.org/doi/10.1146/annurev.aa.30.090192.001523 |journal=[[Annual Review of Astronomy and Astrophysics]] |language=en |volume=30 |issue=1 |pages=311–358 |doi=10.1146/annurev.aa.30.090192.001523 |issn=0066-4146}}</ref>

==== Other AGNs ====
[[Blazar]]s are believed to be active galaxies with a [[relativistic jet]] pointed in the direction of Earth. A [[radio galaxy]] emits radio frequencies from relativistic jets. A unified model of these types of active galaxies explains their differences based on the observer's position.<ref name="monster" />

Possibly related to active galactic nuclei (as well as [[starburst (astronomy)|starburst]] regions) are [[low-ionization nuclear emission-line region]]s (LINERs). The emission from LINER-type galaxies is dominated by weakly [[ion]]ized elements. The excitation sources for the weakly ionized lines include post-[[Asymptotic giant branch|AGB]] stars, AGN, and shocks.<ref name="heckman1980">{{cite journal
 |last1=Heckman |first1=Timothy M.
 |date=1980
 |url=https://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1980A%26A....87..152H&defaultprint=YES&filetype=.pdf
 |title=An Optical and Radio Survey of the Nuclei of Bright Galaxies
 |journal=[[Astronomy & Astrophysics]]
 |volume=87 |pages=152–164
 |number=1–2
 |bibcode=1980A&A....87..152H
}}</ref> Approximately one-third of nearby galaxies are classified as containing LINER nuclei.<ref name="keel" /><ref name="heckman1980" /><ref name="hoetal1997b">{{cite journal
 |last1=Ho |first1=Luis C.
 |last2=Filippenko |first2=Alexei V.
 |last3=Sargent |first3=Wallace L. W.
 |s2cid=16742031
 |date=1997
 |title=A Search for "Dwarf" Seyfert Nuclei. V. Demographics of Nuclear Activity in Nearby Galaxies
 |journal=[[The Astrophysical Journal]]
 |volume=487
 |issue=2 |pages=568–578
 |bibcode=1997ApJ...487..568H
 |doi=10.1086/304638
|arxiv = astro-ph/9704108 }}</ref>