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VanderL
2004-Feb-21, 09:51 AM
Hi there,

Recently a galaxy (M64) was found to have an outer ring rotating counter to the inner part and now on Astronomy picture of the Day (feb 21 2004) I see another example (NGC 4622) of a galaxy rotating the "other way". How many galaxies are rotating the wrong way? If this happens frequently, how can a merger explain this phenomenon? Wouldn't it be impossible for something of that mass to start rotating in the exact opposite direction?
Cheers.

devilmech
2004-Feb-21, 11:40 AM
First off, I would like to point out that this does not happen frequently, and NGC4622 is indeed suspected the subject of a merger with another galaxy, due to the the lopsidedness of it's spiral arms and observations of the galaxy's core.

It's not known whether the rotation was caused by the merger, or whether the gravitational force of a much larger galaxy passing close to it set the galaxy rotating clockwise. If further observation proves these two leading hypotheses to be inadequate explanations, it will lead to a fundamental rethinking of a lot of what we know about galaxies.

Simply put, we don't have a definitive theory as to why galaxies may rotate clockwise, we don't know how frequently it happens, and as for it being impossible of something that massive to start rotating in the exact opposite direction, we now have evidence that it is not only possible, but has happened at least once.

Even more simply put, there's not very much known about the subject :P

VanderL
2004-Feb-21, 08:28 PM
Thanks Devilmech,
The merger scenario is very much in vogue to explain all kinds of strange observations, how frequent can are these mergers be in a Universe where everything is generally moving away? Computer models are used to show what would happen to merging galaxies, but I find it very hard to believe that merging of galaxies account for all the strange features of galaxies. This "counter"-rotation has been observed twice to my knowledge, but my guess is that everyone just assumes that galaxies rotate with their "arms" trailing. Or has this normal rotation been confirmed for hundreds of galaxies?
Cheers.

devilmech
2004-Feb-21, 11:55 PM
Well, the laws of physics and gravitation dictate that galaxies rotate with their arms trailing. As for merging of galaxies, mergers are more frequent than you think. Our own galaxy, the Milky Way, is currently swallowing whole a small dwarf galaxy. In the future, the Milky Way will absorb the Magellanic Clouds, and in a few billion years, the Milky Way will collide with the Andromeda galaxy, which will probably result in an elliptical galaxy much like M87.