View source for Hoag's Object

{{Short description|Unusual galaxy in the constellation Serpens}}
{{Infobox galaxy
| name     = Hoag's Object
| image    = Hoag's object.jpg
| image_scale = 1.5
| caption  = Hoag's Object, taken by the [[Hubble Space Telescope]] in July 2001
| epoch    = [[J2000]]
| ra       = {{RA|15|17|14.4}}<ref name="ned">
{{cite web
 |title=NED results for Hoag's Object
 |work=NASA/IPAC Extragalactic Database
 |url=http://nedwww.ipac.caltech.edu/cgi-bin/nph-objsearch?objname=Hoag%27s+Object&extend=no&hconst=73&omegam=0.27&omegav=0.73&corr_z=1&out_csys=Equatorial&out_equinox=J2000.0&obj_sort=RA+or+Longitude&of=pre_text&zv_breaker=30000.0&list_limit=5&img_stamp=YES
 |access-date=2016-06-28
}}</ref>
| dec      = {{DEC|+21|35|08}}<ref name="ned" />
| constellation name = [[Serpens Caput]]
| z        = {{val|12740|50|u=km/s}}<ref name="ned" /><ref name="OConnelletal1974" />
| dist_ly  = {{val|612.8|9.4|u=[[light-year|Mly]]}} ({{val|187.9|2.9|ul=Mpc}})<ref name="OConnelletal1974" />{{Ref_label|A|a|none}}
| size     = {{convert|45.41|kpc|ly|-3|abbr=off|lk=on}}<ref name="ned" /><ref name=Adelman-McCarthy>{{cite journal|arxiv=0707.3413 |doi=10.1086/524984 |title=The Sixth Data Release of the Sloan Digital Sky Survey |date=2008 |last1=Adelman-Mccarthy |first1=Jennifer K. |last2=Agüeros |first2=Marcel A. |last3=Allam |first3=Sahar S. |last4=Allende Prieto |first4=Carlos |last5=Anderson |first5=Kurt S.J. |last6=Anderson |first6=Scott F. |last7=Annis |first7=James |last8=Bahcall |first8=Neta A. |last9=Bailer-Jones |first9=C. A. L. |last10=Baldry |first10=Ivan K. |last11=Barentine |first11=J. C. |last12=Bassett |first12=Bruce A. |last13=Becker |first13=Andrew C. |last14=Beers |first14=Timothy C. |last15=Bell |first15=Eric F. |last16=Berlind |first16=Andreas A. |last17=Bernardi |first17=Mariangela |last18=Blanton |first18=Michael R. |last19=Bochanski |first19=John J. |last20=Boroski |first20=William N. |last21=Brinchmann |first21=Jarle |last22=Brinkmann |first22=J. |last23=Brunner |first23=Robert J. |last24=Budavári |first24=Tamás |last25=Carliles |first25=Samuel |last26=Carr |first26=Michael A. |last27=Castander |first27=Francisco J. |last28=Cinabro |first28=David |last29=Cool |first29=R. J. |last30=Covey |first30=Kevin R. |journal=The Astrophysical Journal Supplement Series |volume=175 |issue=2 |pages=297–313 |bibcode=2008ApJS..175..297A |display-authors=1 }}</ref><br />(diameter; D<sub>25</sub> isophote)
| type     = (RP)E0 or (RP)SA0/a<ref>{{cite journal |bibcode=2017MNRAS.471.4027B |title=Galactic rings revisited - I. CVRHS classifications of 3962 ringed galaxies from the Galaxy Zoo 2 Database |journal=Monthly Notices of the Royal Astronomical Society |volume=471 |issue=4 |pages=4027–4046 |last1=Buta |first1=Ronald J. |year=2017 |arxiv=1707.06589 |doi=10.1093/mnras/stx1829 |doi-access=free }}</ref>
| appmag_b = 16.2<ref name="ned" />
| size_v   = 0.28′ × 0.28′<ref name="ned" />
| notes    = Ring galaxy
| names    = [[Principal Galaxies Catalogue|PGC]] 54559,<ref name="ned" /> PRC D-51<ref name="ned" />
}}

'''Hoag's Object''' is an unusual [[ring galaxy]] in the constellation of Serpens Caput.<ref name="LS-20191203">{{cite news |last=Specktor |first=Brandon |title=Hoag's Object Is a Galaxy Within a Galaxy Within a Galaxy (and Nobody Knows Why) |url=https://www.livescience.com/hoags-object-perfect-ring-mystery.html |date=3 December 2019 |work=[[Live Science]] |access-date=3 December 2019 }}</ref> It is [[Galaxies named after people|named]] after [[Arthur Hoag]], who discovered it in 1950 and identified it as either a [[planetary nebula]] or a [[peculiar galaxy]].<ref name="Hoag1950">{{cite journal
 | author=A.A. Hoag
 | date=1950
 | title=A peculiar object in Serpens
 | journal=[[Astronomical Journal]]
 | volume=55 | pages=170
 | bibcode=1950AJ.....55Q.170H
 | doi=10.1086/106427
| doi-access=free
 }}</ref> The galaxy has a [[Galaxy#Isophotal diameter|
D<sub>25</sub> isophotal diameter]] of {{convert|45.41|kpc|ly|-3|abbr=off|lk=on}}.<ref name="ned" />

==Characteristics==
A nearly perfect ring of young hot blue [[star]]s circles the older yellow nucleus of this ring galaxy c. 600 million [[light-year]]s away in the constellation [[Serpens]]. The ring structure is so perfect and circular that it has been referred to as "The most perfect ring galaxy".<ref name="Broschetal2013" /> The diameter of the 6 [[arcsecond]] inner core of the galaxy is about {{val|17|0.7|u=kly}} ({{val|5.3|0.2|u=kpc}}) while the surrounding ring has an inner 28″ diameter of {{val|75|3|u=kly}} ({{val|24.8|1.1|u=kpc}}) and an outer 45″ diameter of {{val|121|4|u=kly}} ({{val|39.9|1.7|u=kpc}}).<ref name="OConnelletal1974">
{{cite journal
 |author1=R.W. O'Connell |author2=J.D. Scargle |author3=W.L.W. Sargent |date=1974
 |title=The Nature of Hoag's Object
 |journal=[[Astrophysical Journal]]
 |volume=191 |pages=61–62
 |doi=10.1086/152940
 |bibcode=1974ApJ...191...61O
|doi-access=free}}</ref> The galaxy is estimated to have a mass of 700 billion suns.<ref>{{cite journal |bibcode=1987ApJ...320..454S |title=The Structure and Evolution of Hoag's Object |journal=The Astrophysical Journal |volume=320 |pages=454 |last1=Schweizer |first1=Francois |last2=Ford |first2=W. Kent |last3=Jedrzejewski |first3=Robert |last4=Giovanelli |first4=Riccardo |year=1987 |doi=10.1086/165562 |doi-access=free }}</ref> By comparison, the [[Milky Way]] galaxy has an estimated diameter of 150-200 kly and consists of between 100 and 500 billion stars and a mass between 800 billion and 1.54 trillion suns.<ref>{{Cite web|url=https://www.theguardian.com/science/2019/mar/07/scientists-milky-way-weighs-galaxy-hubble-nasa|title = Scientists discover what the Milky Way weighs|website = [[TheGuardian.com]]|date = 7 March 2019}}</ref><ref>{{Cite web|url=https://www.nasa.gov/feature/goddard/2019/what-does-the-milky-way-weigh-hubble-and-gaia-investigate|title=What Does the Milky Way Weigh? Hubble and Gaia Investigate|date=6 March 2019}}</ref>

The gap separating the two stellar populations may contain some [[star cluster]]s that are almost too faint to see. Though ring galaxies are rare, another more distant ring galaxy (SDSS J151713.93+213516.8)<ref name="SkyServer_2015">{{cite web|url=http://skyserver.sdss.org/dr7/en/tools/explore/obj.asp?id=587739719767949564|title=SkyServer Object Explorer – SDSS J151713.93+213516.8}}</ref> can be seen through Hoag's Object, between the nucleus and the outer ring of the galaxy, at roughly the one o'clock position in the image shown here.

[[Noah Brosch]] and colleagues showed that the luminous ring lies at the inner edge of a much larger neutral hydrogen ring.<ref name="Broschetal2013">
{{cite journal|author1=N. Brosch|author2=I. Finkelman|author3=T. Oosterloo|author4=G. Jozsa|author5=A. Moiseev|date=2013|title=HI in HO: Hoag's Object revisited|journal=[[Monthly Notices of the Royal Astronomical Society]]|volume=435|issue=1|pages=199–206|arxiv=1307.6368|bibcode=2013MNRAS.435..475B|doi=10.1093/mnras/stt1348|doi-access=free }}</ref>

==History and formation==

Even though Hoag's Object was clearly shown on the [[Palomar Star Survey]], it was not included in either the ''[[Morphological Catalogue of Galaxies]]'', the ''[[Catalogue of Galaxies and Clusters of Galaxies]]'', or the ''[[catalogue of galactic planetary nebulae]]''.<ref name="OConnelletal1974" />

In the initial announcement of his discovery, Hoag proposed the hypothesis that the visible ring was a product of [[gravitational lensing]].  This idea was later discarded because the nucleus and the ring have the same [[redshift]], and because more advanced telescopes revealed the ring's knotty structure, which would not be visible if the galaxy were a product of gravitational lensing.<ref name="Schweizeretal1987">
{{cite journal
 |author1=F. Schweizer |author2=W.K. Ford Jr. |author3=R. Jederzejewski |author4=R. Giovanelli |date=1987
 |title=The structure and evolution of Hoag's object
 |journal=[[Astrophysical Journal]]
 |volume=320 |pages=454–463
 |doi=10.1086/165562
 |bibcode=1987ApJ...320..454S
|doi-access=free}}</ref>

Many of the galaxy's details remain mysterious, foremost of which is how it formed. So-called "classic" ring galaxies are generally formed by the collision of a small galaxy with a larger disk-shaped galaxy, producing a density wave in the disk that leads to a characteristic ring-like appearance. Such an event would have happened at least 2–3 billion years ago,<ref name="Schweizeretal1987"/> and may have resembled the processes that form [[polar-ring galaxy|polar-ring galaxies]]. However, there is no sign of any second galaxy that would have acted as the "bullet", and the likely older core of Hoag's Object has a very low velocity relative to the ring, making the typical formation hypothesis quite unlikely.<ref name="Schweizeretal1987" /> Observations with one of the most sensitive telescopes have also failed to uncover any faint galaxy fragments that should be observable in a collision scenario. However, a team of scientists that analyzes the galaxy admits that "if the carnage happened more than 3 billion years ago, there might not be any [[detritus]] left to see."<ref>{{Cite web|url=https://www.newscientist.com/article/dn20885-astrophile-saturn-lookalike-galaxy-has-a-murky-past/|title=Astrophile: Saturn-lookalike galaxy has a murky past|website=www.newscientist.com|access-date=2019-10-31}}</ref>

Noah Brosch suggested that Hoag's Object might be a product of an extreme "bar instability" that occurred a few billion years ago in a [[barred spiral galaxy]].<ref name="Brosch1985">
{{cite journal
 |author=N. Brosch
 |date=1985
 |title=The nature of Hoag's object – The perfect ringed galaxy
 |journal=[[Astronomy and Astrophysics]]
 |volume=153 |issue=1 |pages=199–206
 |bibcode=1985A&A...153..199B
}}</ref> Schweizer ''et al''<ref name="Schweizeretal1987" /> claim this is an unlikely hypothesis because the nucleus of the object is [[spheroidal]], whereas the nucleus of a barred spiral galaxy is disc-shaped, among other reasons. However, they admit evidence is somewhat thin for this particular dispute to be settled satisfactorily.  

A few other galaxies share the primary characteristics of Hoag's Object, including a bright detached ring of stars, but their centers are elongated or barred, and they may exhibit some spiral structure. While none matches Hoag's Object in symmetry, these galaxies are known to some as Hoag-type galaxies.<ref>{{Cite web|url=https://www.theregister.co.uk/2017/01/04/rare_ring_shaped_galaxy_discovered/|title=Astroboffins glimpse sighting of ultra-rare circular galaxy|last=at 10:26|first=Gavin Clarke |date=2017-01-04|website=www.theregister.co.uk|language=en|access-date=2019-10-31}}</ref><ref>{{Cite web|url=https://astronomy.com/magazine/weirdest-objects/2015/11/7-hoags-object|title=Weird Object: Hoags Object|author=Bob Berman|date=2015-11-13|website=Astronomy.com|access-date=2019-10-31|archive-date=2022-07-24|archive-url=https://web.archive.org/web/20220724093714/https://astronomy.com/magazine/weirdest-objects/2015/11/7-hoags-object|url-status=dead}}</ref>

==See also==
*[[LEDA 1000714]]

==Notes==
{{refbegin}}
{{Note_label|A|a|none}}1974 O'Connell paper<ref name="OConnelletal1974" /> assumes a [[Hubble constant]] of 75; this figure adjusts for 2013's {{val|67.8|0.77|u=(km/s)/Mpc}}.
{{refend}}

==References==
{{Reflist}}

==External links==
*{{Commonscat-inline|Hoag's Object}}
*[http://heritage.stsci.edu/2002/21/big.html Hubble Space Telescope image of Hoag's Object.] 
*[http://nedwww.ipac.caltech.edu/level5/Rings/Rings17_2.html Accretion Rings] Galactic Rings – Fund. Cosmic Physics, 1996. Vol. 17, pp.&nbsp;95–281
*[http://hubblesite.org/newscenter/archive/releases/2002/21/image/a/ A Wheel within a Wheel (09/05/2002)] News Release Number: STScI-2002-21 – HubbleSite
*[https://web.archive.org/web/20061126190949/http://www.spacetelescope.org/images/html/opo0221a.html ESA/Hubble image of Hoag's Object]
{{WikiSky}}

{{Stars of Serpens}}
{{Portal bar|Astronomy|Stars|Outer space}}
[[Category:Ring galaxies]]
[[Category:Serpens]]
[[Category:Peculiar galaxies]]
[[Category:Principal Galaxies Catalogue objects|54559]]
[[Category:Astronomical objects discovered in 1950|?]]