Polar-ring galaxy

{{short description|Type of galaxy in which an outer ring of gas and stars rotates over the poles of the galaxy}}

File:NGC 660 Polar Galaxy Gemini Observatory.jpg showing polar-galaxy structure]]

A polar-ring galaxy is a type of galaxy with an outer ring of gas and stars that rotates over the poles of the galaxy.{{cite book

| author=James Binney

| author2=Michael Merrifield

| date=1998

| title=Galactic Astronomy

| publisher=Princeton University Press

| location=Princeton, New Jersey

| isbn=0-691-00402-1}} 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.{{cite journal |last1=Ristea |first1=Andrei |title=The SAMI Galaxy Survey: physical drivers of stellar-gas kinematic misalignments in the nearby Universe |journal=Monthly Notices of the Royal Astronomical Society |date=2023 |volume=517 |issue=2 |page=2677 |doi=10.1093/mnras/stac2839 |doi-access=free |url=https://academic.oup.com/mnras/article/517/2/2677/6753231|arxiv=2210.01147 }} 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.{{cite journal

| author=F. Schweizer

| author2=B. C. Whitmore

| author3=V. C. Rubin

| title=Colliding and merging galaxies. II - S0 galaxies with polar rings

| journal=Astronomical Journal

| date=1983

| volume=88

| pages=909–925

| bibcode=1983AJ.....88..909S

| doi=10.1086/113377| doi-access=free

}}

The best-known polar-ring galaxies are S0s (lenticular galaxies), but from the physical point of view they are part of a wider category of galaxies, including several ellipticals.

The first four S0 galaxies that were identified as polar-ring galaxies were NGC 2685,{{cite journal

| author=P. L. Schecter

| author2=J. E. Gunn

| title=NGC 2685 - Spindle or pancake

| journal=Astronomical Journal

| date=1978

| volume=83

| pages=1360–1362

| bibcode=1978AJ.....83.1360S

| doi=10.1086/112324}} NGC 4650A,{{cite journal

| author=J. L. Sérsic

| title=Southern Peculiar Galaxies III

| journal=Zeitschrift für Astrophysik

| date=1967

| volume=67

| pages=306–311

| bibcode=1967ZA.....67..306S}}{{cite journal

| author=B. C. Whitmore

| author2=D. B. McElroy

| author3=F. Schweizer

| title=The shape of the dark halo in polar-ring galaxies

| journal=Astrophysical Journal

| date=1987

| volume=314

| pages=439–456

| bibcode=1987ApJ...314..439W

| doi=10.1086/165077}} A 0136 -0801, and ESO 415 -G26. While these galaxies have been extensively studied, many other polar-ring galaxies have since been identified.{{cite journal

| author= B. C. Whitmore

| author2= R. A. Lucas

| author3= D. B. McElroy

| author4= T. Y. Steiman-Cameron

| author5= P. D. Sackett

| author6= R. P. Olling

| title=New observations and a photographic atlas of polar-ring galaxies

| journal=Astronomical Journal

| date=1990

| volume=100

| pages=1489–1522, 1721–1755

| bibcode=1990AJ....100.1489W

| doi=10.1086/115614| doi-access=free

}} Polar-ring S0 galaxies 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.

The first polar-ring elliptical galaxies were identified in 1978. They were NGC 5128, NGC 5363, NGC 1947 and Cygnus A,{{cite journal

| author = Bertola, F.

| author2 = Galletta, G.

| name-list-style = amp

| title = A new type of galaxy with prolate structure

| journal =Astrophysical Journal

| date = 1978

| volume = 226

| pages = L115–L118

| bibcode =1978ApJ...226L.115B

| doi =10.1086/182844| doi-access = free

}},

while the polar-ring S0 galaxies NGC 2685 and NGC 4650A were at that time indicated as resulting from similar formation processes. Only some years later, when the first observations of the stellar and gas motion of polar-ring elliptical and S0 galaxies were possible with a better spectroscopic technology, the external origin of the gaseous rings was clarified.{{cite journal

| author = Bertola, F.

| author2 = Galletta, G.

| author3 = Zeilinger, W.~W.

| title = Warped dust lanes in elliptical galaxies - Transient or stationary phenomena?

| journal = Astrophysical Journal

| date = 1985

| volume = 292

| pages = L51–L55

| bibcode = 1985ApJ...292L..51B

| doi = 10.1086/184471}}

{{cite journal

| author = Varnas, S.R. Bertola

| author2 = F., Galletta

| author3 = G., Freeman

| author4 = K.C., Carter, D.

| title = NGC 5266 - an elliptical galaxy with a dust ring

| journal = Astrophysical Journal

| date = 1987

| volume = 313

| pages = 69–88

| bibcode = 1987ApJ...313...69V

| doi = 10.1086/164949}}

In addition to the best-known example, NGC 5128 (Cen A), a very regular polar ring elliptical, is NGC 5266.