by flash » Mon Jun 04, 2012 7:43 pm
rstevenson wrote:flash wrote:Given that the two about-to-collide galaxies are spirals, and since spiral galaxies have large net angular momentum (as opposed to elliptical and spherical), then unless the two galaxies have anti-parallel or nearly anti-parallel angular momentum vectors (and nearly equal in magnitude) I would exect the resultant galaxy (after the merge) to also have large angular momentum, and eventually form another spiral. Isn't angular momentum still conserved when galaxies collide? And doesn't that mean the resultant galaxy will be a spiral?
I can see the possibility that two galaxies spinning in the same direction and meeting gently top to bottom might remain a spiral after merging, but that would be an extremely rare type of merger. It's more likely they will be at some angle to each other and, about 50% of the time, will not be spinning in the same direction. So the resulting merger will be
messy (that's a technical term
), and though individual stars and systems will indeed maintain much of their individual momentum, there isn't much reason to assume that the overall momentum will be all pointing in a tidy spiral-type pattern.
Try tossing two pizza doughs in the air and getting them to combine into one larger dough. Messy!
Rob
It doesn't (shouldn't) matter how messy it is: angular momentum is angular momentum: It must be conserved. Just as with linear momentum: If you could somehow get 1million BBs to simultaneously collide with a bowling ball, their combined center of mass will move in exactly the same manner before, during, and after the collision. The same must be true of the angular momentum of the galaxies. Before, during, and after the collision. the combined angular momentum vector must remain constant. I think that means that the result will necessarily be a spiral galaxy (if you wait long enough, and under the assumptions of inital angular momentum alignment, etc)
[quote="rstevenson"][quote="flash"]Given that the two about-to-collide galaxies are spirals, and since spiral galaxies have large net angular momentum (as opposed to elliptical and spherical), then unless the two galaxies have anti-parallel or nearly anti-parallel angular momentum vectors (and nearly equal in magnitude) I would exect the resultant galaxy (after the merge) to also have large angular momentum, and eventually form another spiral. Isn't angular momentum still conserved when galaxies collide? And doesn't that mean the resultant galaxy will be a spiral?[/quote]
I can see the possibility that two galaxies spinning in the same direction and meeting gently top to bottom might remain a spiral after merging, but that would be an extremely rare type of merger. It's more likely they will be at some angle to each other and, about 50% of the time, will not be spinning in the same direction. So the resulting merger will be [i]messy[/i] (that's a technical term :lol2: ), and though individual stars and systems will indeed maintain much of their individual momentum, there isn't much reason to assume that the overall momentum will be all pointing in a tidy spiral-type pattern.
Try tossing two pizza doughs in the air and getting them to combine into one larger dough. Messy!
Rob[/quote]
It doesn't (shouldn't) matter how messy it is: angular momentum is angular momentum: It must be conserved. Just as with linear momentum: If you could somehow get 1million BBs to simultaneously collide with a bowling ball, their combined center of mass will move in exactly the same manner before, during, and after the collision. The same must be true of the angular momentum of the galaxies. Before, during, and after the collision. the combined angular momentum vector must remain constant. I think that means that the result will necessarily be a spiral galaxy (if you wait long enough, and under the assumptions of inital angular momentum alignment, etc)