View source for Centaurus A

{{Short description|Radio galaxy in the constellation Centaurus}}
{{hatnote group|
{{about-confuse|the [[radio galaxy]]|HD 125823{{!}}a Centauri|HD 100673{{!}}A Centauri|Alpha Centauri{{!}}α Centauri}}
{{other uses|Centaurus (disambiguation)}}
}}
{{Infobox galaxy
| name = Centaurus A
| image = [[Image:ESO Centaurus A LABOCA.jpg|300px]]
|caption = Centaurus A (NGC 5128)
| epoch = [[Epoch (astronomy)#Julian years and J2000|J2000]]
| type = S0 pec<ref name="ned">{{cite web
  | title=NASA/IPAC Extragalactic Database
  | work=Results for Centaurus A
  | url=http://nedwww.ipac.caltech.edu/
  | access-date=6 December 2006 }}</ref> or Ep<ref name=harris10b/>
| ra = {{RA|13|25|27.6}}<ref name="ned" />
| dec = {{DEC|-43|01|09}}<ref name="ned" />
| dist_ly = 11–13 [[light-year|Mly]] (3–5 [[parsec|Mpc]])<ref name="tonryetal2001">{{cite journal
  | author=J. L. Tonry | display-authors=4 | author2=A. Dressler | author3=J. P. Blakeslee | author4=E. A. Ajhar | author5=A. B. Fletcher | author6=G. A. Luppino | author7=M. R. Metzger | author8=C. B. Moore
  | title=The SBF Survey of Galaxy Distances. IV. SBF Magnitudes, Colors, and Distances
  | journal=Astrophysical Journal
  | date=2001
  | volume=546
  | issue=2
  | pages=681–693
  | bibcode=2001ApJ...546..681T   | doi=10.1086/318301
|arxiv = astro-ph/0011223 | s2cid=17628238 }}</ref><ref name="ned-dist">{{cite web
  |title=Distance Results for NGC 5128
  |work=NASA/IPAC Extragalactic Database
  |url=http://nedwww.ipac.caltech.edu/cgi-bin/nDistance?name=NGC+5128
  |access-date=26 April 2010}}</ref><ref name=ferrarese07/><ref name=majaess10/><ref name=harris10/>
| z = 547 ± 5&nbsp;km/[[second|s]]<ref name="ned" />
| appmag_v = 6.84<ref name="SIMBAD-A">{{cite simbad
 |title=NAME Centaurus A
 |access-date=29 November 2009}}</ref><ref name=GAXEL>{{cite journal
  |last=Armando |first=Gil de Paz
  |title=The GALEX Ultraviolet Atlas of Nearby Galaxies
  |journal=Astrophysical Journal Supplement
  |volume=173
  |issue=2 |pages=185–255 |date=2007
  |doi=10.1086/516636
  |arxiv=astro-ph/0606440
|bibcode=2007ApJS..173..185G
  |author2=Boissier
  |author3=Madore
  |author4=Seibert
  |author5=Boselli
  |display-authors=5
  |last6=Boselli
  |first6=Alessandro
  |last7=Wyder
  |first7=Ted K.
  |last8=Thilker
  |first8=David
  |last9=Bianchi
  |first9=Luciana
|s2cid=119085482
 }}</ref>
| size_v = 25'.7 × 20'.0<ref name="ned" />
| constellation name = [[Centaurus]]
| notes = Closest [[radio galaxy]]; closest [[BL Lacertae object]]; unusual [[dust lane]]
| names = [[New General Catalogue|NGC]] 5128,<ref name="ned" /> [[Atlas of Peculiar Galaxies|Arp]] 153,<ref name="ned" /> [[Principal Galaxies Catalogue|PGC]] 46957,<ref name="ned" /> 4U 1322–42,<ref>{{cite web|url=http://heasarc.gsfc.nasa.gov/W3Browse/uhuru/uhuru4.html|title=UHURU4 – Uhuru Fourth (4U) Catalog|website=heasarc.gsfc.nasa.gov}}</ref> [[Caldwell catalogue|Caldwell]] 77
|size = 123,000 [[Light-Year|ly]] (37.74 [[kiloparsec|kpc]]) (estimated)<ref name = ned/>}}

'''Centaurus A''' (also known as '''NGC 5128''' or '''Caldwell 77''') is a [[galaxy]] in the [[constellation]] of [[Centaurus]]. It was discovered in 1826 by [[Scotland|Scottish]] astronomer [[James Dunlop]] from his home in [[Parramatta]], in New South Wales, Australia. There is considerable debate in the literature regarding the galaxy's fundamental properties such as its [[Hubble sequence|Hubble type]] ([[lenticular galaxy]] or a giant [[elliptical galaxy]])<ref name=harris10b/> and distance (11–13 million [[light-year]]s).<ref name="tonryetal2001"/><ref name="ned-dist"/><ref name=ferrarese07/><ref name=majaess10/><ref name=harris10/> It is the closest [[radio galaxy]] to [[Earth]], as well as the closest [[BL Lac object]],<ref name=Chiaberge>{{cite journal | bibcode=2001MNRAS.324L..33C | title=The BL Lac heart of Centaurus A | last1=Chiaberge | first1=M. | last2=Capetti | first2=A. | last3=Celotti | first3=A. | journal=Monthly Notices of the Royal Astronomical Society | date=2001 | volume=324 | issue=4 | pages=L33–L37 | doi=10.1046/j.1365-8711.2001.04642.x | doi-access=free | arxiv=astro-ph/0105159 }}</ref> so its [[active galactic nucleus]] has been extensively studied by professional astronomers.<ref name="israel1998">{{cite journal
  | author=F. P. Israel
  | title=Centaurus A – NGC 5128
  | journal=Astronomy and Astrophysics Review
  | date=1998
  | volume=8
  | issue=4
  | pages=237–278
  | bibcode=1998A&ARv...8..237I  | doi=10.1007/s001590050011
|arxiv = astro-ph/9811051 | s2cid=18353347
 }}</ref> The galaxy is also the [[List_of_galaxies#Naked-eye_galaxies|fifth-brightest]] in the sky,<ref name="israel1998"/> making it an ideal amateur astronomy target.<ref name="eicher1988">{{cite book
  | author=D. J. Eicher
  | date=1988
  | title=The Universe from Your Backyard
  | publisher=Cambridge University Press
  | location=Cambridge
  | isbn=978-0-521-36299-3
  | url-access=registration
  | url=https://archive.org/details/universefromyour0000eich
  }}</ref> It is only visible from the southern hemisphere and low northern latitudes.

The center of the galaxy contains a [[supermassive black hole]] with a mass of 55 million [[solar mass]]es,<ref>{{cite web|title=Radio Telescopes Capture Best-Ever Snapshot of Black Hole Jets|date=20 May 2011 |url=http://www.nasa.gov/topics/universe/features/radio-particle-jets.html|publisher=NASA|access-date=2 October 2012}}</ref> which ejects a [[Astrophysical jet#Relativistic jet|relativistic jet]] that is responsible for emissions in the [[X-ray]] and [[Radio spectrum|radio]] wavelengths.  By taking radio observations of the jet separated by a decade, astronomers have determined that the inner parts of the jet are moving at about half of the [[speed of light]]. X-rays are produced farther out as the jet collides with surrounding gases, resulting in the creation of highly energetic particles. The X-ray jets of Centaurus A are thousands of light-years long, while the radio jets are over a million light-years long.<ref>{{Cite APOD|title=Centaurus Radio Jets Rising|access-date=16 April 2011|date=13 April 2011}}</ref>

It is also one of the nearest large [[Starburst galaxy|starburst galaxies]], of which a [[galaxy merger|galactic collision]] is suspected to be responsible for an intense burst of star formation. Models have suggested that Centaurus A was a large elliptical galaxy that collided with a smaller spiral galaxy, with which it will eventually merge.<ref name=Quillen2006>{{Cite journal | doi = 10.1086/504418| title = Spitzer ''Observations'' of the Dusty Warped Disk of Centaurus A| journal = Astrophysical Journal| volume = 645| issue = 2| pages = 1092–1101| year = 2006| last1 = Quillen | first1 = A. C. | last2 = Brookes | first2 = M. H. | last3 = Keene | first3 = J. | last4 = Stern | first4 = D. | last5 = Lawrence | first5 = C. R. | last6 = Werner | first6 = M. W. |arxiv = astro-ph/0601135 |bibcode = 2006ApJ...645.1092Q | s2cid = 18443587}}</ref> For that reason, the galaxy has been of particular interest to astronomers for years. While collisions of spiral galaxies are relatively common, the effects of a collision between an elliptical and a spiral galaxy are not fully understood.<ref>{{Cite web |url=https://www.eso.org/public/images/eso0005b/ |title=Radio galaxy Centaurus A |publisher=European Southern Observatory (ESO) |date=8 February 2000}}</ref>

== Observational history ==
NGC&nbsp;5128 was discovered on 29 April 1826 by [[James Dunlop]] during a survey at the Parramatta Observatory.<ref>{{Cite journal |last1=Robertson |first1=Peter |last2=Cozens |first2=Glen |last3=Orchiston |first3=Wayne |last4=Slee |first4=Bruce |last5=Wendt |first5=Harry |date=2010 |title=Early Australian Optical and Radio Observations of Centaurus A |url=https://www.cambridge.org/core/product/identifier/S1323358000000503/type/journal_article |journal=Publications of the Astronomical Society of Australia |language=en |volume=27 |issue=4 |pages=402–430 |arxiv=1012.5137 |bibcode=2010PASA...27..402R |doi=10.1071/AS09071 |issn=1323-3580 |s2cid=54580482}}</ref><ref>{{Cite journal |last=Dunlop |first=James |date=1828-12-31 |title=A catalogue of nebulæ and clusters of stars in the southern hemisphere, observed at Paramatta in New South Wales, … |journal=Philosophical Transactions of the Royal Society of London |language=en |volume=118 |pages=113–151 |doi=10.1098/rstl.1828.0010 |issn=0261-0523 |doi-access=free}}
Centaurus A is listed on p. 138 as entry number 482.  A sketch of Centaurus A appears as Fig. 20 on the plate between pages 114 and 115.</ref>

In 1847 [[John Herschel]] described the galaxy as "two semi-ovals of elliptically formed nebula appearing to be cut asunder and separated by a broad obscure band parallel to the larger axis of the nebula, in the midst of which a faint streak of light parallel to the sides of the cut appears."<ref>{{Cite book |last=Herschel |first=John F. W. |author-link=John Herschel |url=https://archive.org/details/outlinesofastron00hersuoft/mode/2up |title=Outlines of astronomy |date=1878 |publisher=London Longmans, Green |isbn=978-1-108-01377-2 |edition=1 |pages=602 |doi=10.1017/cbo9780511709111}}</ref>

In 1949 [[John Gatenby Bolton]], Bruce Slee and Gordon Stanley localized NGC&nbsp;5128 as one of the first extragalactic radio sources.<ref>{{cite journal
   |author1=J. G. Bolton |author2=G. J. Stanley |author3=O. B. Slee |title=Positions of Three Discrete Sources of Galactic Radio-Frequency Radiation
   |journal=Nature
   |volume=164
   |issue=4159
   |date=1949
   |pages=101–102 |doi=10.1038/164101b0|bibcode = 1949Natur.164..101B |s2cid=4073162 }}</ref> Five years later, [[Walter Baade]] and [[Rudolph Minkowski]] suggested that the peculiar structure is the result of a merge event of a giant elliptical galaxy and a small spiral galaxy.<ref>{{cite journal|title=On the Identification of Radio Sources.|first1=W.|last1=Baade|first2=R.|last2=Minkowski|date=1 January 1954|journal=Astrophysical Journal|volume=119|pages=215|via=NASA ADS|doi=10.1086/145813|bibcode=1954ApJ...119..215B}}</ref> The first detection of X-ray emissions, using a sounding rocket, was performed in 1970.<ref>{{cite journal
   |author1=C. S. Bowyer |author2=M. Lampton |author3=J. Mack |author4=F. de Mendonca |title=Detection of X-Ray Emission from 3C&nbsp;273 and NGC&nbsp;5128
   |journal=Astrophysical Journal
   |volume=161
   |date=1970
   |doi=10.1086/180559 |bibcode=1970ApJ...161L...1B |page=L1|url=http://urlib.net/8JMKD3MGP3W34P/3L6DNS2 }}</ref> In 1975–76 gamma-ray emissions from Centaurus A were observed through the [[IACT|atmospheric Cherenkov technique]].<ref>{{cite journal|title=Evidence for the detection of gamma rays from Centaurus A at gamma-ray energies above 300 GeV|first1=J. E.|last1=Grindlay|first2=H. F.|last2=Helmken|first3=R. H.|last3=Brown|first4=J.|last4=Davis|first5=L. R.|last5=Allen|date=1 April 1975|journal=Astrophysical Journal Letters|volume=197|pages=L9–L12NASA ADS|doi=10.1086/181764|bibcode=1975ApJ...197L...9G}}</ref>

The [[Einstein Observatory]] detected an X-ray jet emanating from the nucleus in 1979.<ref>{{cite journal
   |author1=E. J. Schreier |author2=E. Feigelson |author3=J. Delvaille |author4=R. Giacconi |author5=D. A. Schwartz |title=EINSTEIN Observations of The X-Ray Structure of Centaurus&nbsp;A: Evidence For The Radio-Lobe Energy Source
   |journal=Astrophysical Journal, Part 2
   |volume=234
   |date=1979
   |pages=39–43
   |doi=10.1086/183105 |bibcode=1979ApJ...234L..39S|doi-access=free}}</ref> Ten years later, young blue stars were found along the central dust band with the Hubble Space Telescope.<ref>{{cite web
   |title=HubbleSite – NewsCenter – Hubble Provides Multiple Views of How to Feed a Black Hole (05/14/1998) – Background Info
   |url=http://hubblesite.org/newscenter/archive/releases/1998/14/background/
   |access-date=14 October 2015
   |archiveurl=https://web.archive.org/web/20071030030708/http://hubblesite.org/newscenter/archive/releases/1998/14/background/
   |archivedate=2007-10-30}}</ref>

The [[Chandra X-ray Observatory]] identified in 1999 more than 200 new point sources.<ref>{{cite journal
   |author1=R. P. Kraft |author2=J. M. Kregenow |author3=W. R. Forman |author4=C. Jones |author5=S. S. Murray |title=Chandra Observations of the X-Ray Point Source Population in Centaurus&nbsp;A
   |journal=Astrophysical Journal
   |volume=560
   |issue=2
   |date=20 October 2001
   |pages=675–688
   |doi=10.1086/323056
   |bibcode=2001ApJ...560..675K|doi-access=
   }}</ref> Another space telescope, the [[Spitzer Space Telescope]], found a parallelogram-shaped structure of dust in near infrared images of Centaurus A in 2006.<ref>{{cite journal
   |author1=Alice C. Quillen |author2=Mairi H. Brookes |author3=Jocelyn Keene |author4=Daniel Stern |author5=Charles R. Lawrence |title=Spitzer Observations of the Dusty Warped Disk of Centaurus&nbsp;A
   |journal=Astrophysical Journal
   |volume=645 Nummer = 2
   |issue=2 |date=10 July 2006
   |pages=1092–1101
   |doi=10.1086/504418
   |bibcode=2006ApJ...645.1092Q|arxiv = astro-ph/0601135 |s2cid=18443587 }}</ref>

Evidence of gamma emissions with very high energy (more than 100&nbsp;GeV) was detected by the [[High Energy Stereoscopic System|H.E.S.S-Observatorium]] in Namibia in 2009.<ref>{{cite journal
   |author1=F. Aharonian |author2=A. G. Akhperjanian |author3=G. Anton |author4=U. Barres de Almeida |author5=A. R. Bazer-Bachi |title=DISCOVERY OF VERY HIGH ENERGY γ-RAY EMISSION FROM CENTAURUS&nbsp;A WITH H.E.S.S.
   |journal=Astrophysical Journal
   |volume=695
   |issue=1
   |date=10 April 2009
   |pages=L40–L44
   |doi=10.1088/0004-637x/695/1/l40
   |bibcode=2009ApJ...695L..40A|arxiv = 0903.1582 |s2cid=18160745 }}</ref>

The following year, Centaurus A was identified as a source of cosmic rays of highest energies, after years of observations by [[Pierre Auger Observatory]].<ref>{{cite journal
   |author1=J. Abraham |author2=P. Abreu |author3=M. Aglietta |author4=C. Aguirre |author5=D. Allard |title=Correlation of the highest-energy cosmic rays with the positions of nearby active galactic nuclei
   |journal=Astroparticle Physics
   |volume=29
   |issue=3
   |date=1 April 2008
   |pages=188–204
   |doi=10.1016/j.astropartphys.2008.01.002
   |arxiv = 0712.2843 |bibcode = 2008APh....29..188P }}</ref> In 2016 a review of data from Chandra and [[XMM-Newton]], unusual high flares of energy were found in NGC 5128 and the galaxy [[NGC 4636]]. [[Jimmy Irwin]] of [[University of Alabama]] hypothesized the discovery as potentially a black hole in a yet unknown process or an [[intermediate-mass black hole]].{{citation needed|date=December 2020}}

==Morphology==
Centaurus A may be described as having a [[Peculiar galaxy|peculiar]] morphology.  As seen from Earth, the galaxy looks like a [[lenticular galaxy|lenticular]] or [[elliptical galaxy]] with a superimposed dust lane.<ref name="carnegieatlas">{{cite book
  | author=A. Sandage | author2=J. Bedke
  | date=1994
  | title=Carnegie Atlas of Galaxies
  | publisher=Carnegie Institution of Washington
  | location=Washington, D.C.
  | isbn=978-0-87279-667-6}}</ref> The peculiarity of this galaxy was first identified in 1847 by [[John Herschel]], and the galaxy was included in [[Halton Arp]]'s ''[[Atlas of Peculiar Galaxies]]'' (published in 1966) as one of the best examples of a "disturbed" galaxy with dust absorption.<ref name="arp1966">{{cite journal
  | author=H. Arp
  | title=Atlas of Peculiar Galaxies
  | journal=Astrophysical Journal Supplement
  | date=1966
  | volume=14
  | pages=1–20
  | bibcode=1966ApJS...14....1A  | doi=10.1086/190147
}}</ref> The galaxy's strange morphology is generally recognized as the result of a [[interacting galaxy|merger]] between two smaller galaxies.<ref name="baademinkowski1954">{{cite journal
  | author=W. Baade | author2=R. Minkowski
  | title=On the Identification of Radio Sources
  | journal=Astrophysical Journal
  | date=1954
  | volume=119
  | pages=215–231
  | bibcode=1954ApJ...119..215B
  | doi=10.1086/145813}}</ref>

[[File:Centaurus A EN.webm|thumb|Zoom movie of the galaxy Centaurus A, showing different aspects of the galaxy in several wavelengths.]]
[[File:StuctureCenA-EN.png|thumb|Schematic diagram of the components of the Centaurus A galaxy]]
The bulge of this galaxy is composed mainly of evolved red stars.<ref name="carnegieatlas" /> The dusty disk, however, has been the site of more recent [[star formation]];<ref name="israel1998"/> over 100 star formation regions have been identified in the disk.<ref name="hodgekennicutt1983">{{cite journal
  | author=P. W. Hodge | author2=R. C. Kennicutt Jr.
  | title=An atlas of H II regions in 125 galaxies
  | journal=Astrophysical Journal
  | date=1982
  | volume=88
  | pages=296–328
  | bibcode=1983AJ.....88..296H
  | doi=10.1086/113318}}</ref>

==Novae and supernovae==
[[File:Supernova 1986 G in the peculiar, southern galaxy Centaurus A (NGC 5128) (eso8608a).jpg|thumb|left|SN 1986G imaged with the ESO 40-cm double astrograph on La Silla]]Two [[supernova]]e have been detected in Centaurus A.<ref name="nedsn">{{cite web
  | title=NASA/IPAC Extragalactic Database
  | work=Results for extended name search on Centaurus A
  | url=http://nedwww.ipac.caltech.edu/
  | access-date=7 March 2007}}</ref>
The first supernova, named [[SN 1986G]], was discovered within the dark dust lane of the galaxy by [[Robert Evans (astronomer)|Robert Evans]] on 3 May 1986.<ref name="iauc">{{cite journal
  | author=R. Evans | author2=R. H. McNaught | author3=C. Humphries
  | title=Supernova 1986G in NGC 5128
  | journal=IAU Circular
  | date=1986
  | pages=1
  | volume=4208
  | bibcode=1986IAUC.4208....1E
  }}</ref>
It was later identified as a [[Type Ia supernova]],<ref name="phillipsetal1987">{{cite journal
  | author=M. M. Phillips | display-authors=4 | author2=A. C. Phillips | author3=S. R. Heathcote | author4=V. M. Blanco | author5=D. Geisler | author6=D. Hamilton | author7=N. B. Suntzeff | author8=F. J. Jablonski | author9=J. E. Steiner | author10=A. P. Cowley | author11=P. Schmidtke | author12=S. Wyckoff | author13=J. B. Hutchings | author14=J. Tonry | author15=M. A. Strauss | author16=J. R. Thorstensen | author17=W. Honey | author18= J. Maza | author19= M. T. Ruiz | author20= A. U. Landolt | author21= A. Uomoto | author22=R. M. Rich | author23=J. E. Grindlay | author24= H. Cohn | author25=H. A. Smith | author26=J. H. Lutz | author27=R. J. Lavery | author28=A. Saha
  | title=The type 1a supernova 1986G in NGC 5128 – Optical photometry and spectra
  | journal=Publications of the Astronomical Society of the Pacific
  | date=1987
  | volume=99
  | pages=592–605
  | bibcode=1987PASP...99..592P  | doi=10.1086/132020| doi-access= }}</ref>
which forms when a [[white dwarf]]'s mass grows large enough to ignite carbon fusion in its center, touching off a [[thermal runaway|runaway]] thermonuclear reaction, as may happen when a white dwarf in a [[binary star]] system strips gas away from the other star. SN 1986G was used to demonstrate that the spectra of type Ia supernovae are not all identical, and that type Ia supernovae may differ in the way that they change in brightness over time.<ref name="phillipsetal1987" />

The second supernova, designated SN 2016adj,<ref>{{cite web|url=http://www.virtualtelescope.eu/2016/02/09/supernova-sn-2016adj-in-the-peculiar-galaxy-ngc-5128-centaurus-a-9-feb-2016/|title=Supernova SN 2016adj in the peculiar galaxy NGC 5128 ("Centaurus A") – 9 Feb. 2016 – The Virtual Telescope Project 2.0|date=9 February 2016}}</ref> was discovered by Backyard Observatory Supernova Search in February 2016 and was initially classified as a Type II supernova based on its  H-alpha [[emission line]].<ref>{{cite web|url=http://www.astronomerstelegram.org/?read=8651|title=ATel #8651: Bright PSN in NGC5128 (Centaurus A) Discovered By Backyard Observatory Supernova Search (BOSS)|website=ATel}}</ref><ref>{{cite web|url=http://www.astronomerstelegram.org/?read=8655|title=ATel #8655: Tentative Spectroscopic Classification of Bright PSN in NGC 5128 (Centaurus A)|website=ATel}}</ref>
A subsequent classification found the spectrum best resembled the Type Ib core-collapse supernova 1999dn.<ref>{{cite web|url=http://www.astronomerstelegram.org/?read=8657|title=ATel #8657: CSP Spectroscopic Classification of AT 2016adj in NGC 5128 (Centaurus A)|website=ATel}}</ref>  (See [[Type Ib and Ic supernovae]]).

In addition to these supernovae, a [[luminous red nova]], designated AT{{nbsp}}2020nqq (type ILRT, mag. 17.8), was discovered on 27 June 2020.<ref>{{cite web | website=Transient Name Server | title=AT{{nbsp}}2020nqq | url=https://www.wis-tns.org/object/2020nqq | publisher = [[International_Astronomical_Union|IAU]] | access-date=5 September 2024}}</ref>

Centaurus A is close enough that classical [[nova]]e can also be detected.  The first confirmed nova in this galaxy was discovered by [[BlackGEM]] at magnitude 18.47 on 23 December 2024, and designated AT 2024aeql.<ref>{{cite web | website=Transient Name Server | title=AT{{nbsp}}2024aeql | url=https://www.wis-tns.org/object/2024aeql | publisher = [[International Astronomical Union|IAU]] | access-date=24 December 2024}}</ref>

==Distance==
Distance estimates to Centaurus A established since the 1980s typically range between 3–5 Mpc.<ref name="tonryetal2001"/><ref name="ned-dist"/><ref name=ferrarese07/><ref name=majaess10/><ref name=harris10/><ref name=rejkuba04/> [[Classical Cepheids]] discovered in the heavily obscured dust lane of Centaurus A yield a distance between ~3–3.5 Mpc, depending on the nature of the extinction law adopted and other considerations.<ref name=ferrarese07>{{cite journal | author = Ferrarese Laura | display-authors = 4 | author2 = Mould Jeremy R. | author3 = Stetson Peter B. | author4 = Tonry John L. | author5 = Blakeslee John P. | author6 = Ajhar Edward A. | date =2007 | title = The Discovery of Cepheids and a Distance to NGC 5128 | doi = 10.1086/506612 | bibcode=2007ApJ...654..186F | journal = Astrophysical Journal | volume = 654 | issue = 1| pages = 186–218 |arxiv = astro-ph/0605707 | s2cid = 17895798 }}</ref><ref name=majaess10>{{cite journal|bibcode=2010AcA....60..121M|author1=Majaess, D.|date=2010|pages=121|volume=60 |issue=2|title=The Cepheids of Centaurus A (NGC 5128) and Implications for H0 |journal=Acta Astronomica|arxiv = 1006.2458 }}</ref> [[Mira variable]]s<ref name=rejkuba04>{{cite journal |bibcode=2004A&A...413..903R |title=The distance to the giant elliptical galaxy NGC 5128 |journal=Astronomy and Astrophysics|arxiv = astro-ph/0310639 |doi = 10.1051/0004-6361:20034031 |date=2004 |last1=Rejkuba |first1=M. |volume=413 |issue=3 |pages=903–912 |s2cid=15938660 }}</ref> and [[Type II Cepheid]]s<ref name=ferrarese07/><ref name=majaess10/> were also discovered in Centaurus A, the latter being rarely detected beyond the [[Local Group]].<ref name=majaess09>{{cite journal |author1=Majaess, D. |bibcode=2009AcA....59..403M |date=2009 |author2=Turner, D. |page=403 |author3=Lane, D. |title=Type II Cepheids as Extragalactic Distance Candles |volume=59 |issue=4 |journal=Acta Astronomica|arxiv = 0909.0181 }}</ref> The distance to Centaurus A established from several indicators such as [[Mira variable]]s and [[planetary nebula]]e favour a more distant value of ~3.8 Mpc.<ref name=harris10b>{{cite journal |bibcode=2010PASA...27..475H |author1=Harris, Gretchen L. H. |doi=10.1071/AS09063 |title=NGC 5128: The Giant Beneath |date=2010 |pages=475–481 |issue=4 |volume=27 |journal=Publications of the Astronomical Society of Australia|arxiv = 1004.4907 |s2cid=118604663 }}</ref><ref name=harris10>{{cite journal |doi=10.1071/AS09061 |last1=Harris |arxiv=0911.3180 |first1=Gretchen L. H. |last2=Rejkuba |first2=Marina |last3=Harris |first3=William E. |title=The Distance to NGC 5128 (Centaurus A) |journal=Publications of the Astronomical Society of Australia |volume=27 |issue=4 |pages=457–462 |date=2010|bibcode = 2010PASA...27..457H |s2cid=16912328 }}</ref>

==Nearby galaxies and galaxy group information==
Centaurus A is at the center of one of two subgroups within the [[Centaurus A/M83 Group]], a nearby [[Galaxy group|group of galaxies]].<ref name="karachentsevetal2002">{{cite journal
  | author=I. D. Karachentsev | display-authors=4 | author2=M. E. Sharina | author3=A. E. Dolphin | author4=E. K. Grebel | author5=D. Geisler | author6=P. Guhathakurta | author7=P. W. Hodge | author8=V. E. Karachetseva | author9=A. Sarajedini | author10=P. Seitzer
  | title=New distances to galaxies in the Centaurus A group
  | journal=Astronomy and Astrophysics
  | date=2002
  | volume=385
  | issue=1
  | pages=21–31
  | bibcode=2002A&A...385...21K  | doi=10.1051/0004-6361:20020042
| doi-access=free }}</ref> [[Messier 83]] (the Southern Pinwheel Galaxy) is at the center of the other subgroup.  These two groups are sometimes identified as one group<ref name="nbg">{{cite book
  | author=R. B. Tully
  | date=1988
  | title=Nearby Galaxies Catalog
  | publisher=Cambridge University Press
  | location=Cambridge
  | isbn=978-0-521-35299-4}}</ref><ref name="fouqueetal1992">{{cite journal
  | author=P. Fouque | author2=E. Gourgoulhon | author3=P. Chamaraux | author4=G. Paturel
  | title=Groups of galaxies within 80 Mpc. II – The catalogue of groups and group members
  | journal=Astronomy and Astrophysics Supplement
  | date=1992
  | volume=93
  | pages=211–233
  | bibcode=1992A&AS...93..211F
  }}</ref> and sometimes identified as two groups.<ref name="garcia1993">{{cite journal
  | author=A. Garcia
  | title=General study of group membership. II – Determination of nearby groups
  | journal=Astronomy and Astrophysics Supplement
  | date=1993
  | volume=100
  | pages=47–90
  | bibcode=1993A&AS..100...47G}}</ref> However, the galaxies around Centaurus A and the galaxies around M83 are physically close to each other, and both subgroups appear not to be moving relative to each other.<ref name="karachentsev2005">{{cite journal
  | author=I. D. Karachentsev
  | title=The Local Group and Other Neighboring Galaxy Groups
  | journal=Astronomical Journal
  | date=2005
  | volume=129
  | issue=1
  | pages=178–188
  | bibcode=2005AJ....129..178K  | doi=10.1086/426368
|arxiv = astro-ph/0410065 | s2cid=119385141
 }}</ref> The Centaurus A/M83 Group is located in the [[Virgo Supercluster]].{{cn|date=April 2021}}

In addition to dwarf galaxies, Centaurus A, like most galaxies, has a population of [[globular cluster]]s. Some objects that appear to be globular clusters are hypothesized to be the tidally stripped cores of former galaxies. The most extreme example is the object VHH81-01, whose central black hole is estimated to be around {{solar mass|{{val|8e5}}}}.<ref>{{cite journal|doi=10.3847/1538-4357/ac551c|title=A Population of Luminous Globular Clusters and Stripped Nuclei with Elevated Mass to Light Ratios around NGC 5128 |year=2022 |last1=Dumont |first1=Antoine |last2=Seth |first2=Anil C. |last3=Strader |first3=Jay |last4=Voggel |first4=Karina |last5=Sand |first5=David J. |last6=Hughes |first6=Allison K. |last7=Caldwell |first7=Nelson |last8=Crnojević |first8=Denija |last9=Mateo |first9=Mario |last10=Bailey |first10=John I. |last11=Forbes |first11=Duncan A. |journal=The Astrophysical Journal |volume=929 |issue=2 |page=147 |arxiv=2112.04504 |bibcode=2022ApJ...929..147D |s2cid=245005855 |doi-access=free }}</ref>

== Observations ==
=== Radio waves ===
{{wide image|RadioCenA-EN.png|600px|Overview over the radio structure of Centaurus&nbsp;A. The whole radio emitting region extends about 1.8&nbsp;million light years (about 8° degrees in the sky). Through observations with the VLBI technique structures of the jet and the core smaller than a light year could be resolved (corresponding to a resolution of 0.68&nbsp;x&nbsp;0.41&nbsp;milli-arcseconds.<ref>{{cite journal|author1=Roopesh Ojha |author2=M. Kadler |author3=M. Böck |author4=R. Booth |author5=M. S. Dutka |title=TANAMI: Milliarcsecond Resolution Observations of Extragalactic Gamma-ray Sources |journal=2009 Fermi Symposium |date=30 December 2009 |arxiv=1001.0059|bibcode = 2010arXiv1001.0059O }}</ref>) }}

[[File:CentaurusA VR37 hi.tif|thumb|This view of the jets of Centaurus&nbsp;A was created through observations in radio waves with a wavelength of 20&nbsp;cm with the VLA. The position of the radio jet and the knots within the jets matches very well with the structures seen in the x-ray jet. This region of the jet is named „Inner Lobe“.<ref>''[http://images.nrao.edu/11 Inner Radio Lobes of Centaurus&nbsp;A (NGC&nbsp;5128).] {{Webarchive|url=https://web.archive.org/web/20160916110004/http://images.nrao.edu/11 |date=16 September 2016 }}''</ref>]]

In July 2021 the [[Event_Horizon_Telescope#Centaurus_A|Event Horizon Telescope]] released a resolved image of Centaurus A showing the jet coming from the black hole at its center.

==Visibility==

Centaurus A is located approximately 4° north of [[Omega Centauri]] (a [[globular cluster]] visible with the naked eye).<ref name="eicher1988"/> Because the galaxy has a high surface brightness and relatively large angular size, it is an ideal target for amateur astronomy observations.  The bright central bulge and dark dust lane are visible even in finderscopes and large binoculars,<ref name="eicher1988"/> and additional structure may be seen in larger telescopes.<ref name="eicher1988"/> Claims have been made that Centaurus A is visible to the naked eye under exceptionally good conditions.<ref>{{cite web|url=http://astronomy-mall.com/Adventures.In.Deep.Space/aintno.htm|title=Aintno Catalog|website=astronomy-mall.com}}</ref>

== Gallery ==
<gallery perrow="5">
Centaurus A halo.jpg|Wide field view of Centaurus A showing its extended halo of stars
Radio galaxy Centaurus A by ALMA.jpg|The radio galaxy Centaurus A, as seen by [[Atacama Large Millimeter Array|ALMA]]
CentaurusA3.jpg|The heavily obscured inner (barred?) spiral disk at 24 μm as shown by the [[Spitzer Space Telescope|Spitzer IR telescope]]
NGC 5128.jpg|[[Chandra X-ray Observatory|Chandra X-ray]] view of Cen A in X-rays showing one [[relativistic jet]] from the central [[black hole]]
Firestorm of Star Birth in Galaxy Centaurus A.jpg|"Hubble's panchromatic vision... reveals the vibrant glow of young, blue star clusters..."<ref>{{cite web|title=Firestorm of Star Birth in Galaxy Centaurus A|url=http://www.nasa.gov/multimedia/imagegallery/image_feature_2192.html|publisher=NASA |date=March 7, 2012|access-date=27 September 2012}}</ref>
CenA-nearIR.jpg|Image of the central parts of Centaurus A showing the parallelogram-shaped remains of a smaller galaxy that was absorbed about 200 to 700 million years ago
Centauros a-spc.png|"[[False-colour]] image of Centaurus A, showing [[radio wave|radio]] (red), 24-micrometre [[infrared]] (green) and 0.5–5 [[keV]] [[X-ray]] emission (blue)
CentaurusA Center EN.PNG|Central part of the galaxy
Centaurus A jets.ogv|Video about Centaurus A jets
Centaurus a Mark Johnston.jpg|Amateur astronomer photo of Centaurus A
</gallery>

== See also ==
* [[Messier 87]] – a giant elliptical galaxy that is also a strong radio source
* [[NGC 1316]] – a similar lenticular galaxy that is also a strong radio source
*[[NGC 1106]] - another lenticular galaxy with an active galactic nucleus

==References==
{{Reflist|30em}}

==Sources==
* STScI. [http://hubblesite.org/newscenter/newsdesk/archive/releases/1998/14/text/ Hubble Provides Multiple Views of How to Feed a Black Hole]. Press release: ''Space Telescope Science Institute''. 14 March 1998.
* Chandra X-Ray Observatory Photo Album [http://chandra.harvard.edu/photo/2003/cenajet/ Centaurus A Jet]

==External links==
{{Commons category}}
* [http://www.extragalactic.info/cena/cen-a-home.html Sarka Wykes Centaurus A project] {{Webarchive|url=https://web.archive.org/web/20170706220818/http://www.extragalactic.info/cena/cen-a-home.html |date=6 July 2017 }}
* [http://messier.seds.org/xtra/ngc/n5128.html '''SEDS''': Peculiar Galaxy NGC 5128]
* [http://www.spacetelescope.org/images/archive/freesearch/5128/viewall/1 ESA/Hubble images of Centaurus A]
* [http://apod.gsfc.nasa.gov/apod/ap030705.html '''NASA's APOD''': X-Rays from an Active Galaxy (July 5, 2003)]
* [http://apod.gsfc.nasa.gov/apod/ap060304.html '''NASA's APOD''': The Galaxy Within Centaurus A (March 4, 2006)]
* [https://apod.nasa.gov/apod/ap180712.html '''NASA's APOD''': Centaurus A (July 12, 2018)]
* [https://apod.nasa.gov/apod/ap230503.html '''NASA's APOD''': Centaurus A: A Peculiar Island of Stars (May 3. 2023)]
* [http://www.universetoday.com/wp-content/uploads/2006/07/ngc5128.jpg High-resolution image of Centaurus A] showing the discrete elements of galactic core
* [http://www.universetoday.com/?s=Centaurus Centaurus A] at UniverseToday.com
* [https://web.archive.org/web/20090209000355/http://www.dsi-astronomie.de/Centaurus.html NGC5128 Centaurus A]
* [http://www.docdb.net/show_object.php?id=ngc_5128 NGC 5128] at '''DOCdb''' (Deep Sky Observer's Companion)
* {{WikiSky}}
* [http://www.constellation-guide.com/centaurus-a/ Centaurus A at Constellation Guide]


{{Sky|13|25|27.6|-|43|01|09|13700000}}

{{catalogs | NGC=5128 | Arp=153 | PGC=46957 | C=77}}
{{Caldwell catalogue}}
{{Ngc55}}
{{Centaurus}}
{{Portal bar|Astronomy|Stars|Outer space}}
{{Use dmy dates|date=September 2019}}

{{DEFAULTSORT:Centaurus A}}
[[Category:Lenticular galaxies]]
[[Category:Peculiar galaxies]]
[[Category:Radio galaxies]]
[[Category:Starburst galaxies]]
[[Category:BL Lacertae objects]]
[[Category:Centaurus A/M83 Group]]
[[Category:Centaurus]]
[[Category:NGC objects]]
[[Category:Principal Galaxies Catalogue objects|46957]]
[[Category:Arp objects|153]]
[[Category:Caldwell objects|077b]]
[[Category:Polar-ring galaxies]]
[[Category:Astronomical objects discovered in 1826]]
[[Category:Articles containing video clips]]
[[Category:Galaxy mergers]]