APOD: Arp 81: 100 Million Years Later (2014 Apr 23)

[Ody]

I am no physicist and certainly no astronomer so my question may seem a little banal so please don't crucify me and here goes anyway.
The description of that comes with today’s most excellent photo makes it pretty clear the two galaxies are merging. However, given we have been observing this phenomena for mere millionths if not billionths of a second in evolutionary time, how can we be certain they are merging and not tearing apart? As a galaxy spins, aren’t there centrifugal type forces that could in fact tear a galaxy apart? Which would be the greater, gravity, which I accept would pull them together, or centrifugal, that could tear apart?

[Nitpicker]

I am no physicist and certainly no astronomer so my question may seem a little banal so please don't crucify me and here goes anyway.
The description of that comes with today’s most excellent photo makes it pretty clear the two galaxies are merging. However, given we have been observing this phenomena for mere millionths if not billionths of a second in evolutionary time, how can we be certain they are merging and not tearing apart? As a galaxy spins, aren’t there centrifugal type forces that could in fact tear a galaxy apart? Which would be the greater, gravity, which I accept would pull them together, or centrifugal, that could tear apart?

There are no centrifugal forces, only attractive gravitational forces between masses. (There are other forces as well, but gravity is by far the most dominant on a galactic scale.) The things that might cause a galaxy to tear apart include:

a) if gravity suddenly stopped working (this seems unlikely).

b) some other galaxy sweeping through and perturbing things with its own gravitational forces. If perturbed sufficiently, some parts of the galaxies may be torn apart for good, but most parts will probably continue interacting with each other.

[DavidLeodis]

On searching the APOD archive it seems to me that the image used for the APOD of 2003 December 11 looks to be the same as that used by Martin Pugh to produce his processed version. The detail that has been brought out by Martin is excellent.

This should be the link to the APOD of 2003 December 11 http://apod.nasa.gov/apod/ap031211.html

[scotch7@gmail.com]

We model Galaxies in collision and assume that there is no actual merging of stars or black holes.

Is that realistic?
(Given that two colliding galaxies will contain 300Bn to 800Bn stars and at least two super massive black holes).

Have any been detected?

If such collisions are likely to occur, what might they look like to an observer from Earth?

[Chris Peterson]

We model Galaxies in collision and assume that there is no actual merging of stars or black holes.

Is that realistic?

Yes. The volume occupied by stars is an incredibly small part of the volume of the galaxies as a whole.