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Amongst the most spectacular and rare galaxies known are the so-called "ring galaxies", the best known examples being the Cartwheel Galaxy, UGC 7069, and Hoag's object (large image above).
Today, 128 ring galaxies are known, most of them in the relatively nearby Universe, within 1 billion light years.
Of all these, that discovered most recently, called Auriga's Wheel (inset), is perhaps one of the most interesting, because it catches "live" the mechanism that causes the ring structure, that is, a perpendicular impact of a smaller galaxy through the central regions of a larger spiral galaxy.
During an event of this type, gravitational perturbations generate density waves that propagate outwards in the larger galaxy, compressing the interstellar medium and triggering the formation of new stars.
Just like the waves made by a stone thrown into a pond, these density waves cause the star formation process to propagate outward in an ever growing concentric region.
Until now the dynamics of the phenomenon had only been extrapolated from the reciprocal positions of the two galaxies involved, always observed at large spatial and temporal separations from the actual collision. But now, astronomers at the Max Planck Institute for Astronomy, during a study of the thick disk of our own galaxy, have captured a ring structure in the process of formation, with the "bullet" galaxy still very close to its target.
Spectroscopic analysis of the light coming from the two components of Auriga's Wheel confirm that they are at the same distance, about 1.5 billion light years, a record for a galaxy of this type. The ring is expanding at a velocity of 200 km/s, which implies that the collision occurred 50 million years ago. Both nuclei of the two galaxies are active, but it is not clear whether this is a consequence of the interaction or if they were active before the impact.
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