Polar-Ring Galaxies

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Polar-ring galaxies (PRGs) are among the most interesting and mysterious examples of interactions between galaxies. Accretion, either from tidal capture of matter from a nearby system or the merger of a gas-rich companion, is generally considered as an explanation of the observed structure of PRGs. Whitmore et al.(1990) (AJ, vol.100, p.1489) have compiled the "Catalog of polar-ring galaxies, candidates and related objects" (PRC) listing 157 known and suspected PRGs. See the catalog: 1 2 3 4 5

The unique geometry of PRGs makes them laboratories for a several important astrophysical studies:

they offer an ideal opportunity to test the 3-D shape of the galaxy potentials, since the polar rings extend out to many scale-lengths of the inner disk;

physical characteristics of the ring (total luminosity, stellar composition, HI, H_{2}, and dust masses etc.) give a direct way to estimate characteristics of a merged or accreted galaxy. The kinematic and morphological features of the polar ring (its orientation, thickness, and position) provide a record of the dynamics of the interaction itself (rotation axis orientation, relative speed, impact parameter) which would be of use in galaxy interaction models;

these objects are useful for our understanding of the accretion induced nuclear non-thermal activity and nuclear starbursts;

the rings represent an unusual setting for star formation, since they represent coherent structures which do not necessarily possess the global triggers found in the disks of spiral galaxies, or the compression into a deep potential well, as in starburst galaxies;

it seems that polar rings formation and evolution imitate formation and evolution of the galactic disks in some aspects. Therefore, detailed study of polar rings provides a new approach to early stages of galactic evolution.

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Thie figure presents numerical simulation of the formation of a polar ring during the interaction of two galaxies (see F.Bournaud, F.Combes, 2003, A&A, v.401, p.817-833 and this page). At the beginning, the future host galaxy (at the center) is a normal spiral galaxy. Another galaxy (only gas clouds of the galaxy are shown) interacts with it, even if no merger occurs. Tidal forces are so strong that a part of the second galaxy is captured by the host galaxy and winds up around the host disk, to form the polar ring. The companion galaxy, after having lost most of its gas, escapes and runs away from the host.

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We present here a gallery of PRGs and candidates images (in the Johnson B or V passbands mainly) obtained at the 6-meter telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences, and also in Brasil (1.6-m) and France (1.2-m). All the images are in false colours in order to stress unusual morphology of the galaxies.

UGC 7576 ( PRC A-4 )

NGC 4650A ( PRC A-5 ) (ESO VLT photo); NGC 4650A (HST image)

UGC 9796 ( PRC A-6 )

AM 1934-563 ( PRC B-18 ) (1.6-m telescope of the OPD, Brasil) - August 2002

ESO 603-G21 ( PRC B-21 ) (1.6-m telescope of the OPD, Brasil)

A 2330-3751 ( PRC B-23 ) (1.6-m telescope of the OPD, Brasil) - August 2002

ESO 474-G26 ( PRC C-3 ) (1.6-m telescope of the OPD, Brasil) - August 2002

UGC 1198 ( PRC C-12 )

NGC 660 ( PRC C-13 ) (SDSS image)

UGC 4261 ( PRC C-24 )

UGC 4385 ( PRC C-27 )

UGC 5101 ( PRC C-30 )

UGC 10205 ( PRC C-50 ) (1.2-m telescope of the OHP, France)

NGC 6286 ( PRC C-51 )

NGC 7468 ( PRC C-69 ) (1.2-m telescope of the OHP, France)

NGC 3808 ( PRC D-19 )

(to be continued)

Candidate polar-ring galaxies in the Hubble Deep Field:

2-809 (image size is 3x6.6 arcsec)

2-906 (image size is 3.2x3.5 arcsec)

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Additional PRGs sites (with some images) are:

F.Bournaud, F.Combes

Douglas B. McElroy

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