Zlender Images

Polar Ring Galaxies

Polar ring galaxies are an extremely rare specimen among normal lenticular S0-galaxies (see  classification of galaxies ). Only 3 out of 10,000 (Vera Rubin, 1982,83) possess such a peculiar feature. The existence of two nearly orthogonal orbital planes can not be explained from a single formation process. A likely explanation might be an encounter of a small, gas-rich galaxy in a polar orbit about the larger S0-galaxy. In the course of time the smaller one will be tidally disrupted, forming the observable ring. The survival  of such a structure, made of gas, dust and stars is due to the fact, that S0-galaxies have no dust and gas content in the plane of their disk, like normal spirals have. If it were present, it would brake the streaming motion of the gas and dust in the ring while it crosses the plane of the disk. This is the secret why allmost all of the hundred polar ring galaxies known to date, are made of a S0-galaxy as the major component. Computations of G.F.Simonson of Yale University showed, that long-lasting rings can only exist within about 10° from the poles. Otherwise precession of the ring will shorten it's lifetime dramatically. The determination  of the dynamical properties in the ring enables astronomers to weight the encircled mass of the parent galaxy. Again it is found, that this mass must be much more than the mass that can be deduced from the luminous matter.

  NGC2685, Polar ring galaxy in UMa, 05/18/99, jpeg, 8k
 

I am pleased to notice, that my image shows the ring more clearly than the Palomar DSS image of NGC 2685.  Enlarged for more detail.

NGC 660, shown below, contradicts this model of polar ring galaxies, but it must be emphasized that the ring in this case contains much more mass and it's diameter is much larger than in the 'normal'  case.

Adjust your monitor to see the faint outer features.

  NGC660 ,Large distorted galaxy in Pisces, 12/17/98, jpeg, 10k

In a preprint from W. van Driel et.al., Nobeyama Radio Observatory, Japan (NRO), 1994, I found the following interesting details :  "NGC 660 is a unique, nearby object among the rare class of polar ring galaxies, since it has a polar ring with an unusual  morphology and  a gas-rich inner disk with a nuclear LINER spectrum, while other objects of this class usually have a quiescent lenticular-type disk. It has two distinct morphological and kinematic components: a disk, viewed almost edge-on (i~70°), with a major axis position angle of 45° and a diameter of  ~11kpc [D=13 Mpc, H0=75km s-1 Mpc-1],and an outer ring (p.a. 170°) with a diameter of 31kpc, inclined on average 55° with respect to the disk major axis. Though not truly 'polar' (i.e. perpendicular to the disk), it can still be regarded as such for all practical purposes, and we will therefore refer to it as a polar ring. The ring appears to be rather inclined and warped ..."
 

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