1)Does the Local Group (of galaxies) rotate as a whole?
2)Does it revolve around anything? 3)Is it moving as a package in the general expansion of the universe?
Hi!
This should answer your questions.
welcome to the SDMB!
1-2) I think KarlGauss’ link described it well although I did not see a discussion about the revolution of the Local Group.
I have not heard whether the Local Group has any revolution around a common center of mass (presumably located somewhere between the Milky Way and the Andromeda Galaxies - - the 2 largest members of the Local Group), although I suspect it would be possible.
On the other hand, the Milky Way and Andromeda are the big players in the Local Group…and they’re apparently on a collision course. So, perhaps that would be more of a factor than any overall revolution (i.e., the collision would happen long before 1 revolution of the Group around the center of mass). Now that I write it, this rings a bell for some reason…so I think there is no significant overall revolution of the Group.
- Yes. And as noted above and in the link, there is a lot more interaction among the Local Group galaxies than there is any expansion between them. The Milky Way is currently (or will soon) making a meal of the Magellanic Clouds (2 small irregular galaxies).
a couple of my references…
http://image.gsfc.nasa.gov/poetry/ask/a11855.html
http://image.gsfc.nasa.gov/poetry/ask/q2889.html
I do not know if the Local Group rotates although it is likely. However, the Local Group is part of the Local Super cluster of thousands of galaxies centered on the Virgo Cluster, roughly 100 million light years away. The gravitational attraction of this cluster should produce a rotation around its “center” (assuming that we are not, at this moment, plunging to our galactic death at the core of this cluster.)
Galaxies in our (as well as other) Supercluster appear to lie roughly in the same plane. These sheet-like structures are on the scale of hundreds of millions of light years. (The “sheets” are separated by even more enormous voids, thus producing the “bubble” analogy.) This planar orientation might be the result of rotation.
One astronomer has proposed physical associations of super clusters, a billion light years across, also lying in a plane.
Of course, the planar structure might be the result of unknown factors occurring at the time of galactic formation.
Just to allay everyone’s fears, we are not on a collision course with m31, AKA the Andromeda galaxy. The two galaxies are currently approaching each other, but not head-on: We’re just on the approaching portion of our elliptical orbit. That should answer the original question, as well: There is, indeed, a net rotation. Most of the other galaxies in the local group orbit one or the other of the two spirals separately, but the spirals (Milky Way and m31) are enough larger than them that they don’t really matter much.
In any stable structure in the Universe (short of the Universe itself, if you consider that stable), there’s always rotation. If there weren’t, the structure would collapse under gravity, and it wouldn’t be stable.
An item in the most recent Sky & Telescope indicates that the Milky Way has about 2 trillion solar masses of dark matter while M31 is a relative lightweight with 1.2 trillion solar masses. That’s in addition to the visible matter (stars and nebulae).
The most recent survey of galaxies has definitely established that the supercluster is the largest organization of matter in the universe. The largest superclusters are about 250 million lightyears across, although most are smaller than that.
This survey is called 2dF and so far has measured some 100,000 galaxies to a depth of 4 billion lightyears. When done it will have measured 250,000 galaxies.