Polar-ring galaxy

A polar ring galaxy is a rare type of galaxy, created by the merger of two galaxies.

Classification

Polar ring galaxies are as Gezeitenarm galaxies, interacting galaxies and other dwarf galaxies as special types of galaxies with respect to the shape. They consist of a central elongated object and a ring or a disk of stars is perpendicular to this object, ie, over the pole ' of the seen from the side window of the central object. The central objects are mostly lenticular type S0 in the Hubble sequence, but also elliptical or spiral galaxies occur as central objects. The axes of rotation of central object and ring are nearly perpendicular to each other. Other types of galaxies forms ring structures are ring galaxies and some barred spiral galaxies, in individual cases, the distinction can sometimes be difficult.

It has been discovered to date only about 100 polar ring galaxies. There are sub-types with a large rather spherical central object. They are called Saturn type. A contrasting variant is called ( see Figure NGC 4650 ) floppy sombrero type or type. In this form of the ring is relatively larger and heavier.

Today one has evidence that the Milky Way has a small consisting of gas clouds perpendicular to its plane oriented ring. However, the mass of this ring is small and thus not sufficient to be classified as a polar ring galaxy.

Formation

In the formation of a galaxy of gas in a single collapse operation, a gas disk would emerge with a preferred direction of rotation, the collapse participating gas clouds with different sense of rotation would be aligned by collisions with the general rotation. A formation in a train of polar ring galaxies with their mutually perpendicular rotation of the ring and central galaxy is therefore unlikely.

Polar ring galaxies are therefore seen as the result of a subsequent merger of two galaxies. In today's cosmology one assumes that galaxies ' hierarchical ' arise from smaller units and merge again. Depending on the properties of the two merging galaxies and their relative path itself can form end products of very different qualities, under favorable conditions, but may be in a merger the gas a smaller galaxy instead of being completely absorbed by the larger in a stable path perpendicular to the disk of the put more massive central galaxy and form a polar ring galaxy.

Known polar ring galaxies

NGC 4650A

One of the best-studied polar ring galaxy NGC 4650A is 150 million light years away. This galaxy is often regarded as the prototype of the class. The central part contains older yellowish stars, almost perpendicular to rotate a much larger ring (really a large disk as a thin ring ) with younger blue stars.

Hoag's object

Main article: Hoag's Object

The 600 million light years distant polar ring galaxy Hoag's object in the constellation of the Serpent was discovered in 1950 by astronomer Art Hoag. She has a stable and well-defined ring with many hot stars. The radius of the ring is about 120,000 light-years. The center contains many older stars and has a spherical shape.

After the discovery of this then-unknown type of galaxy the discoverer suspected an optical illusion by a gravitational lens as an explanation for their unusual shape. Since later the same redshift was measured for the central object and the ring, this declaration is no longer in question. In addition, soon showed through better telescopes the clusters in the ring, which could not arise from a gravitational lens.

It is still not clear yet whether Hoag's Object is a seen from a different direction polar ring galaxy. Earlier theories suggested a development of an unstable barred spiral galaxy. The spherical shape of the center said, but on the other hand, as barred spirals have flat centers. Since it is a stable structure, it is assumed here today a collision of galaxies. Since this has probably already happened two or three billion years ago, it is only difficult to prove.

Polar Ring Galaxies and Dark Matter

One of the first indications of the existence of dark matter were the rotation curves of spiral galaxies. The high rotational speeds at greater distances from the nucleus can not be explained solely by the visible matter. From the rotation curves in the disk of a galaxy but you can not see, such as the halo dark matter is distributed perpendicular to the plane of the galaxy. Polar ring galaxies in principle allow a measurement of this distribution as the vertical circumferential to the Galaxy Matter of the ring taking, test particles ' provides. In practice, however, the analysis of these measurements difficult, yet so there is no clear picture has emerged such as round, flattened or elongated this dark halo.

Swell

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