Galaxies in Pegasus (APOD 24 Nov 2007)

[iampete]

About the "Stephan's Quintet" colliding galaxies -

Roughly how far are the galactic centers from one another, in terms of the galactic radii themselves? Just looking at a 2-D picture can be deceiving.

I'm aware of several observations of two galaxies in "collision". On the surface, the likelihood of five galaxies in close enough proximity to one another to have "collisions" would seem to be miniscule. Is this a unique observation, or are there other examples of multiple galaxies in such close proximity to one another?

Thanks.

[stargazer_7000]

hi Iampete,

well, stephan's quintet consits of NGC 7317, 7318A/B, 7319, and 7320.
they are said to be located in a distance of abour 250 mio lightyears away and they display gravimetric interaction with one another - except ngc 7320. she is supposed to be a "true" member of the deerlick group, interacting with ngc 7331. actually it is much closer to us, at a distance of some 100 mio lightyears.
hope this answers your questions a bit.
best regards,
dietmar
http://www.stargazer-observatory.com

[Qev]

At a back-of-the-envelope calculation, the centers of the closest two interacting galaxies are roughly 33000 lightyears apart.

[iampete]

I guess I wasn't clear what I was trying to get at (I'm not sure if I know what I'm trying to get at, either!!)

Are these galaxies all part of a "galactic group" or whatever the correct terminology is, i.e., with relatively small relative motion to one another, or are they just meeting at the same place at the same time just by happenstance? It would seem that in the former case, they'd eventually coalesce into one big galaxy, while in the latter case, they'd be likely to just eventually go their separate ways, after a lot of matter exchanges & deformations. If they're all part of a galactic group and will eventually merge, is this thought to be a "normal" process of galactic evolution, or do galaxies, once formed, pretty much remain separate?

If I understand correctly, the so-called "dark energy" manifests itself as a repulsive force. Since the matter exchanges and deformations of "colliding" galaxies are gravitation induced phenomena, it would seem that even at larger-than-galactic scales, the attractive forces are still stronger than the repulsive ones. In the theory that dark energy will eventually result in "infinite" expansion of the universe, at what scale does the repulsive force begin to overwhelm the gravitational force?

And yes, I'm aware that the time scales are too large for us to ever confirm the processes by observation, but I'm curious what the thinking on this is. >>> in layman's terms, please, if possible !!!!!

[Chris Peterson]

Are these galaxies all part of a "galactic group" or whatever the correct terminology is, i.e., with relatively small relative motion to one another, or are they just meeting at the same place at the same time just by happenstance?

They are a true grouping, i.e. they are gravitationally bound.

It would seem that in the former case, they'd eventually coalesce into one big galaxy

No more than all the planets of the Solar System will eventually coalesce with the Sun, or all the stars in a galaxy will coalesce. The galaxies in a group orbit about each other. Such orbits will be unstable, so you get collisions, but there's nothing that can generally cause them all to end up as one galaxy.

while in the latter case, they'd be likely to just eventually go their separate ways, after a lot of matter exchanges & deformations.

To separate, a galaxy would need to achieve escape velocity. There's no way for it to do that. Some parts of a galaxy, or individual stars might do so by stealing angular momentum, but not an entire galaxy.

In the theory that dark energy will eventually result in "infinite" expansion of the universe, at what scale does the repulsive force begin to overwhelm the gravitational force?

Much greater than the few million light year distances between members of a galaxy cluster.

[iampete]

No more than all the planets of the Solar System will eventually coalesce with the Sun, or all the stars in a galaxy will coalesce. The galaxies in a group orbit about each other. Such orbits will be unstable, so you get collisions, but there's nothing that can generally cause them all to end up as one galaxy.

It seems to me that these are dichotomous. A solar system (or a single galaxy by itself) represents the stable "end state" (if there really is such a thing) of a process of gravitational interactions among the constituents. A group of continually colliding galaxies cannot be considered a stable "end state".

Every time galaxies "collide", there should be immense energy losses involved as various portions of each swing through each others' gas/dust clouds, etc. (APOD often identifies various EM emission phenomena as resulting from various gas cloud interactions.) If the galaxies form a continuously interacting and colliding group, then, eventually, these losses should be sufficient to result in a stable, minimum energy configuration without further deformations/collisions. I was picturing the mass transfer from the less massive star to the more massive star in some binary star systems as the mechanism which would result in a single large galaxy.

[Chris Peterson]

It seems to me that these are dichotomous. A solar system (or a single galaxy by itself) represents the stable "end state" (if there really is such a thing) of a process of gravitational interactions among the constituents. A group of continually colliding galaxies cannot be considered a stable "end state".

To be completely accurate, multiple body systems are almost never stable. That includes galaxies and solar systems- although these are generally stable over billions of years.

Every time galaxies "collide", there should be immense energy losses involved as various portions of each swing through each others' gas/dust clouds, etc.

"Immense energy" by human standards, perhaps, but tiny compared with the total kinetic energy of the systems. When two galaxies collide, the vast majority of the matter isn't much affected. The primary force acting on close galaxies is tidal- effects are locally very small, but the overall shape of the galaxies can be altered significantly.

If the galaxies form a continuously interacting and colliding group, then, eventually, these losses should be sufficient to result in a stable, minimum energy configuration without further deformations/collisions.

That's probably true- but during the long period of multiple collisions, a good deal of material will probably be lost to the system. And "eventually" is likely a very long time. I doubt the Universe has been around long enough to allow most galaxy clusters to coalesce... although there are some odd galaxies that are believed to have swallowed up nearby galaxies.

[iampete]

Thanks for your explanations.

I appreciate your patience in answering dumb questions.

[starnut]

No more than all the planets of the Solar System will eventually coalesce with the Sun, or all the stars in a galaxy will coalesce. The galaxies in a group orbit about each other. Such orbits will be unstable, so you get collisions, but there's nothing that can generally cause them all to end up as one galaxy.

Chris,

I thought that over time galaxies in collision would eventually merge into one giant ellipitical galaxy and their central black holes would either merge as well or end up orbiting each other.

[BMAONE23]

Chris,

I thought that over time galaxies in collision would eventually merge into one giant ellipitical galaxy and their central black holes would either merge as well or end up orbiting each other.

Sufficient angular momentum could cause the orbital momentum to reach equilibrium and cause a sustained co-orbital situation.

It might even be that this type of interaction causes the expulsion of material in jets and gives them their corkscrew appearance.