Galaxies come in a wide range of forms. Among these are some especially beautiful examples with prominent ring structures. These galactic exceptions have varied causes.

THE MORPHOLOGY OF RING GALAXIES

Like the irregular and interacting galaxies, ring galaxies are galactic exceptions whose history has played a deciding role. They are the result of collisions of two galaxies which led to something new – a ring galaxy. In addition to a bright nucleus, this type of galaxy has a ring, which is usually symmetrically oriented about the centre. The size of the ring greatly exceeds the size of the central component. This ring is very prominent in photos since it contains bright starforming regions with young, massive stars. One usually recognizes a blue ring which contrasts in colour with the yellowish central component. The colours of these components remind one of a spiral galaxy with its bulge and the spiral arms, thus a ring is comparable to a closed spiral arm. This morphological peculiarity was explicitly taken into account by de Vaucouleurs in his classification scheme: “(R)” is placed at the beginning of the description when a ring is seen. When the morphology of the ring is observed in more detail, differences from galaxy to galaxy become apparent. One thus distinguishes two cases: a) the ring formed via a genuine collision of two partner galaxies, b) the ring structure is the result of wound-up tidal tails of two mutually orbiting partner galaxies.

In the latter case, perspective also plays an important role, since tidal tails result in a sufficiently bright ring only when seen one on top of the other. Ring galaxies formed by a collision are often referred to in the literature as “true colliding galaxies”. But in this case as well, a small angle between the plane of the ring and the line of sight can mean that one doesn't recognize the ring, since it is covered by dust regions or parts of the spiral arm. It is assumed that only about 30 per cent of all ring galaxies can be reliably identified.