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[[Image:NGC 4650A I HST2002.jpg|right|thumb|140px|NGC 4650A, an example of a polar-ring galaxy.]]


[[Image:NGC 4650A I HST2002.jpg|right|thumb|140px|[[NGC 4650A]] is one example of a polar-ring galaxy]]
A '''polar-ring galaxy''' is a type of galaxy in which an outer ring of gas and stars rotates over the poles of the galaxy. These polar rings are thought to form when two galaxies gravitationally interact with each other. One possibility is that a material is tidally stripped from a passing galaxy to produce the polar ring seen in the polar-ring galaxy.  The other possibility is that a smaller galaxy collides orthogonally with the plane of rotation of the larger galaxy, with the smaller galaxy effectively forming the polar ring structure.


A '''polar-ring galaxy''' is a type of [[galaxy]] in which an outer ring of gas and stars rotates over the poles of the galaxy.<ref name="binneymerrifield1998"/>{{cite book
The first four galaxies that were identified as polar-ring galaxies were NGC 2685, While these galaxies have been extensively studied, many other polar-ring galaxies have since been identified.
  | author=James Binney, Michael Merrifield
  | year=[[1998]]
  | title=Galactic Astronomy
  | publisher=Princeton University Press
  | location=Princeton, New Jersey
  | id=ISBN 0-691-00402-1}}  Polar rings may be found around 0.5% of all nearby [[lenticular galaxies]], and it is possible that 5% of lenticular galaxies may have had polar rings at some point during their lifetimes.<ref name="whitmoreetal1990">{{cite journal
  | author= B. C. Whitmore, R. A. Lucas, D. B. McElroy, T. Y. Steiman-Cameron, P. D. Sackett, R. P. Olling
  | title=New observations and a photographic atlas of polar-ring galaxies
  | journal=Astronomical Journal
  | year=1990
  | volume=100
  | pages=1489-1522, 1721-1755
  | url=http://adsabs.harvard.edu/abs/1990AJ....100.1489W}}</ref>  The best known polar-ring galaxy is [[NGC 4650A]]. It is theorized that these galaxies are formed by the collision of two separate galaxies. The collision results in a structure composed of two rings of material rotating at an approximate right angle to each other.
 
The first four galaxies that were identified as polar-ring galaxies were [[NGC 2685]],<ref name="schectergunn1978">{{cite journal
  | author=P. L. Schecter, J. E. Gunn
  | title=NGC 2685 - Spindle or pancake
  | journal=Astronomical Journal
  | year=1978
  | volume=83
  | pages=1360-1362
  | url=http://adsabs.harvard.edu/abs/1978AJ.....83.1360S}}</ref> [[NGC 4650A]],<ref name="sersic1967">{{cite journal
  | author=J. L. Sérsic
  | title=Southern Peculiar Galaxies III
  | journal=Zeitschrift für Astrophysik
  | year=1967
  | volume=67
  | pages=306-311
  | url=http://adsabs.harvard.edu/abs/1967ZA.....67..306S}}</ref><ref name="whitmoreetal1987">{{cite journal
  | author=B. C. Whitmore, D. B. McElroy, F. Schweizer
  | title=The shape of the dark halo in polar-ring galaxies
  | journal=Astrophysical Journal
  | year=1987
  | volume=314
  | pages=439-456
  | url=http://adsabs.harvard.edu/abs/1987ApJ...314..439W}}</ref> [[A 0136 -0801]],<ref name="schweizeretal1983">{{cite journal
  | author=F. Schweizer, B. C. Whitmore, V. C. Rubin
  | title=Colliding and merging galaxies. II - S0 galaxies with polar rings
  | journal=Astronomical Journal
  | year=1983
  | volume=88
  | pages=909-925
  | url=http://adsabs.harvard.edu/abs/1983AJ.....88..909S}}</ref>  and [[ESO 415 -G26]].<ref name="whitmoreetal1987" />  While these galaxies have been extensively studied, many other polar-ring galaxies have since been identified.<ref name="whitmoreetal1990"/>
 
==References==
 
<div class="references-small"><references/></div>


==External links==
==External links==
 
* [http://antwrp.gsfc.nasa.gov/apod/ap990510.html APOD: May 10, 1999 - Polar Ring Galaxy NGC 4650A]
* [http://antwrp.gsfc.nasa.gov/apod/ap990510.html An image of NGC 4650A, a Polar Ring Galaxy]
* [http://hubblesite.org/newscenter/archive/releases/1999/16/ Internet Voters Get Two Galaxies in One from Hubble]
* [http://hubblesite.org/newscenter/archive/releases/1999/16/ Hubblesite.org] Retrieved [[January 2007]]
* [http://space.newscientist.com/article/mg19726455.500-x-marks-the-spot-in-dark-matter-web.html?feedId=online-news_rss20 X marks the spot in dark matter web] - Polar ring galaxies offer first-hand evidence of the existence of the cosmic web, ''New Scientist'', 29 February 2008


[[Category:Galaxy morphological types]]
[[Category:Galaxy morphological types]]
 
[[Category:Polar-ring galaxies| ]]
[[de:Polarring-Galaxie]]
{{ntnes}}

Latest revision as of 00:31, 11 April 2009

NGC 4650A, an example of a polar-ring galaxy.

A polar-ring galaxy is a type of galaxy in which an outer ring of gas and stars rotates over the poles of the galaxy. These polar rings are thought to form when two galaxies gravitationally interact with each other. One possibility is that a material is tidally stripped from a passing galaxy to produce the polar ring seen in the polar-ring galaxy. The other possibility is that a smaller galaxy collides orthogonally with the plane of rotation of the larger galaxy, with the smaller galaxy effectively forming the polar ring structure.

The first four galaxies that were identified as polar-ring galaxies were NGC 2685, While these galaxies have been extensively studied, many other polar-ring galaxies have since been identified.

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