Ring galaxy

2009-02-25T18:53:47Z

132.66.234.74:

[[Image:Hoag's object.jpg|right|thumb|Hoag's Object, a ring galaxy.]]
A '''ring galaxy''' is a [[galaxy]] with a ring-like appearance. The ring consists of massive, relatively young blue [[star]]s, which are extremely bright. The central region contains relatively little luminous [[matter]]. Some [[Astronomy|astronomers]] believe that ring galaxies are formed when a smaller galaxy passes through the center of a larger galaxy. Because most of a galaxy consists of empty space, this "collision" rarely results in any actual collisions between stars. However the [[gravity|gravitational]] disruptions caused by such an event could cause a [[wave]] of star formation to move through the larger galaxy. Others think that rings are formed around some galaxies when external accretion takes place. The star formation would then take place in the accreted material because of the shocks and compressions of the accreted material.

[[Hoag's Object]], discovered by [[Art Hoag]] in 1950 is an example of such a galaxy.

==See also==
*[[Interacting galaxy]]

==External links==
*[http://antwrp.gsfc.nasa.gov/apod/ap020909.html Hoag's Object] at [[Astronomy Picture of the Day]].

[[Category:Ring galaxies|*]]
[[Category:Galaxy morphological types]]

{{galaxy-stub}}

[[bg:Пръстеновидна галактика]]
[[de:Ringgalaxie]]
[[es:Galaxia anular]]
[[fr:Galaxie à anneau]]
[[it:Galassia ad anello]]
[[he:גלקסיית טבעת]]
[[hu:Gyűrűs galaxis]]
[[nn:Ringgalakse]]
[[pl:Galaktyka pierścieniowa]]
[[sk:Prstencová galaxia]]
[[sl:Obročasta galaksija]]
[[sr:Прстенаста галаксија]]
[[sv:Ringformig galax]]
[[th:ดาราจักรชนิดวงแหวน]]
[[zh:環星系]]

Archive:Hoag's Object

2008-05-15T09:43:49Z

132.66.234.74: /* History and formation */

{{Galaxy
| name = Hoag's Object
| image = [[Image:Hoag'sobject.jpg|300px]]
|caption = Hoag's Object taken by the [[Hubble Space Telescope]].<br>Courtesy of [[NASA]]/[[ESA]]
| epoch = [[J2000]]
| ra = {{RA|15|17|14.4}}<ref name="ned">{{cite web
| title=NASA/IPAC Extragalactic Database
| work=Results for Hoag's Object
| url=http://nedwww.ipac.caltech.edu/
| accessdate=2006-12-21}}</ref>
| dec = {{DEC|+21|35|08}}<ref name="ned" />
| constellation name = [[Serpens Caput]]
| z = 12,740 ± 50 [[kilometer|km]]/[[second|s]]<ref name="OConnelletal1974">{{cite journal
| author=O'Connell, Robert W.; Scargle, Jeffrey D.; Sargent, W. L. W.
| title=The Nature of Hoag's Object . .
| journal=Astrophysical Journal
| year=1974
| volume=191
| pages=61-62
| url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1974ApJ...191...61O | doi = 10.1086/152940
}}</ref><ref name="ned" />
| dist_ly = 600 ± 30 [[light-year|Mly]] (183 ± 8 [[parsec|Mpc]])<ref name="OConnelletal1974" />{{Ref_label|A|a|none}}
| type = SA0(r){{Fact|date=February 2007}}
| appmag_v = 16.0<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 a non-typical [[galaxy]] of the type known as a [[ring galaxy]]. The appearance of this object has interested amateur astronomers as much as its uncommon structure has fascinated professionals. The galaxy was discovered in [[1950]] by astronomer [[Arthur Allen Hoag]] who identified it as either a [[planetary nebula]] or a [[peculiar galaxy]].<ref name="Hoag1950">{{cite journal
| author=Hoag, Arthur A.
| title=A peculiar object in Serpens
| journal=Astronomical Journal
| year=1950
| volume=55
| pages=170
| url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1950AJ.....55Q.170H
| doi = 10.1086/106427
}}</ref>

==Characteristics==
A nearly perfect ring of young hot blue [[star]]s circle the older yellow nucleus of this ring galaxy ~600 million [[light-year]]s away in the constellation [[Serpens]]. The diameter of the 6″ inner core of the galaxy is about 17 ± 0.7 kly (5.3 ± 0.2 kpc) while the surrounding ring has an inner 28″ diameter of 75 ± 3 kly (24.8 ± 1.1 kpc) and an outer 45″ diameter of 121 ± 4 kly (39.9 ± 1.7 kpc), which is slightly larger than the [[Milky Way Galaxy]].<ref name="OConnelletal1974" />{{Ref_label|A|a|none}} The gap separating the two stellar populations may contain some [[star cluster]]s that are almost too faint to see. As rare as this type of galaxy is, oddly another more distant ring galaxy can be seen, between the nucleus and the outer ring at the 0100 (one o'clock) position.

==History and formation==
Even though Hoag's Object was clearly shown on the [[Palomar Star Survey|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, Art 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 knotty structure of the ring, something that would not be visible if the ring were the product of gravitational lensing.<ref name="Schweizeretal1987">{{cite journal
| author=[[François Schweizer|Schweizer, François]]; Ford, W. Kent, Jr.; Jederzejewski, Robert; Giovanelli, Riccardo
| title=The structure and evolution of Hoag's object
| journal=Astrophysical Journal, Part 1 (ISSN 0004-637X)
| year=[[September 15]] [[1987]]
| volume=320
| pages=454-463
| url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1987ApJ...320..454S
}}</ref>

Many of the details of the galaxy remain a mystery, 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. This collision produces a density wave in the disk which leads to a characteristic ring-like appearance. However, there is no sign of any second galaxy that would have acted as the "bullet", and the core of Hoag's Object has a very low velocity relative to the ring, making the typical formation hypothesis quite unlikely.

Noah Brosch suggested that Hoag's Object might be a product of an extreme "bar instability" which occurred a few billion years ago in a [[barred spiral galaxy]].<ref name="Brosch1985">{{cite journal
| author=[[Noah Brosch|Brosch, Noah]]
| title=The nature of Hoag's object - The perfect ringed galaxy
| journal=Astronomy and Astrophysics (ISSN 0004-6361)
| year=[[December]] [[1985]]
| volume=153
| pages=199-206
| url=http://adsabs.harvard.edu/abs/1985A%26A...153..199B
bibcode=1985A&A...153..199B
}}</ref>
. Schweizer, et al. (1987)<ref name="Schweizeretal1987" /> claim that 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. Interestingly, a few 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 match Hoag's Object in symmetry, this handful of galaxies are known to some as Hoag-type galaxies.

While there is much still to be explained about the formation of Hoag's Object, it is suggested that a major "accretion event", in other words a collision or a capturing of one galaxy by another, caused the formation of the galaxy we see today. The event likely took place at least 2-3 billion years in the past, and may have resembled the processes that form [[polar-ring galaxy|polar-ring galaxies]].

==External links==

* [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
* http://oposite.stsci.edu/pubinfo/PR/2002/21/index.html
*[http://www.spacetelescope.org/images/html/opo0221a.html ESA/Hubble image of Hoag's Object]

== Notes ==
<div class="references-small">
<ol type="a">
<li>{{Note_label|A|a|none}}1974 O'Connell paper<ref name="OConnelletal1974" /> assumes a [[Hubble constant]] of 75 this figure adjusts for 2006's 70{{±|2.4|3.2}} (km/s)/Mpc.</li>
</ol>
</div>

==References==

<div class="references-small"><references/></div>

[[Category:Ring galaxies]]
[[Category:Serpens constellation]]
[[Category:PGC objects|54559]]

[[de:Hoags Objekt]]
[[es:Objeto de Hoag]]
[[fr:Objet de Hoag]]
[[it:Oggetto di Hoag]]
[[lb:Hoag's Objet]]
[[pt:Objeto de Hoag]]
[[sk:Hoagov objekt]]

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Ring galaxy

Archive:Hoag's Object