Ok, this is going to be a long one, but really, it is not all that hard to follow. (assuming for the moment that I get it right, of course)
<P ALIGN=“CENTER”>**Why do Galaxies rotate? ** </P>
Start with a very large number of gas molecules, monatomic hydrogen, a dash of Helium, some ions, free electrons, and just a sprinkle of heavier atoms, all drifting over a very large volume. For the sake of simplicity we will consider our volume to be remote enough from other things to be dealt with as isolated over galaxy lifetimes. The entire cloud is very chaotic, the individual particles acting independently, and moving randomly.
There does exist a point which is the mathematical center of mass of the cloud. All of the particles are attracted to the center of mass, some are moving away from it with more than escape velocity. Those do escape, or interact with other particles, and either do, or don’t retain escape velocity. Among all the chaos, as certain portion of the material does not escape, and therefore, must orbit the mass.
Considered from the center of mass, an infinite number of axises (sp?) can be drawn through the center of mass. For each such axis there are two possibilities: the cloud has a net angular momentum of zero, or not zero. Probability is not overwhelmingly in favor of non-zero, but the existence of any non zero value with respect to any axis is sufficient to allow the next evolutionary step. One axis has the greatest magnitude of angular momentum. The mathematics of this whipped me for a week or so, and I am not up to trying to provide proof. The coincidence of a huge cloud of particles, all moving, and no angular momentum on any axis is really very unlikely.
The local gravity (with respect to the particles within the cloud) is sufficient to overcome outside gravity forces, and is all aligned to the center of the cloud. The particles will interact as they orbit, and the interactions will tend to cause several things to happen. Objects further out from the center will be more consistently attracted toward the center as they are far away, and will accelerate. They will be subject to forces for longer during the outer portions of their orbit, and by a larger number of other particles. The orbits will “regularize” as that happens over time, and become more circular.
Objects in orbital planes at an angle other than 90 to the axis will pass by more objects “below” them as they cover half of their orbit, and more “above” them during the other half, and will interact in a manner which transfers their angular momentum into the plane of the ecliptic more often than out of it. Objects passing near the center of the cloud will have an increasing chance of collision, and close capture with other particles, and aggregates of particles. The interactions of the more massive aggregates will be more common along the ecliptic, and toward the center of the cloud.
As this all happens, the very small rotational speed will gradually increase, just like the old physics class experiment with the kids on the merry-go-round. It will never get all that rapid, only three or four times each billion years. But the overall effect is relentless, and cumulative. The spiral galaxy effect generally is caused by interactions between galaxies, or at least strongly affected by them. The rotation of gas clouds within the galaxies result from the same forces, and end up making the much smaller objects like stars, and planets spin very fast. One planet I know of spins around once a day!
<P ALIGN=“CENTER”> Tris </P>
“Can you do addition?” the White Queen asked. “What’s one and one and one and one and one and one and one and one and one and one?” “I don’t know,” said Alice. “I lost count.”
– Lewis Carroll, Through the Looking Glass
