Ellipticals vs. spirals colliding

[Loren Booda]

Which is more likely to happen for individual events:


two elliptical galaxies colliding to form two spiral galaxies,

or

two spiral galaxies colliding to form two elliptical galaxies?

[bombadil]

Simulations seem to suggest that two spirals (of similar masses) colliding will produce an elliptical galaxy.

[twofish-quant]

What seems to happen is that spirals tend to collide to form ellipticals. You can think of things in this way. Imagine two spinning disks of gas hitting each other. Chances are that when they hit each other they will be tilted against each other, so that when the gas in the two galaxies start hitting each other, they will end up flying in random directions. So what happens is that two discs that hit each other will tend to become a blob. (i.e. an elliptical galaxy)

If you have two blobs hit each other, they are likely to turn into an even bigger blob rather than a disk.

[Loren Booda]

Thanks both.

[DaveC426913]

I am pretty sure that such a complex structure as a disc can only evolve after a lot of time left undisturbed. A major upset in a galaxy or galaxies will result in ellipticals. However, if left long enough, it might then evolve into a spiral.

[twofish-quant]

In fact the disk itself isn't that complex a structure. Once you have a disk then complex structures develop because of density waves within the disk. Something that I've been told happens is that spiral arms give a misleading impression that the parts in between the arms are empty, when that isn't true. What happens is that when you have extra density, that gives you more star formation, and hot blue stars show up better on telescopes.