Bigger galaxies. And stars.
If any galaxies actually collided on August 16, 2011, we won't know about it for some unknown number of millions of years, as the light of the collision reaches us. If we have just today observed that two distant galaxies appear to be colliding, then they collided some number of millions of years AGO, and the light of the event has just reached us.
They collided with another galaxy
The possessive form for the plural noun galaxies is galaxies'.Example: Their galaxies' paths do not coordinate.
An irregular galaxy is a galaxy that doesn't have a specific shape like a pinwheel or an elliptical galaxy.
radiactive zone
when 2 plates collided
Planetesimals.
Not if, not even when: galactic collisions are very,very common. Rarely the galaxies will just pass through the others; more often they will result in some combination of merging and then tossing out the matter that lies at the gravitational perifery.
That's called a supercluster.
Galaxies
It depends on how much gravity that causes stars to form. It depends on how big the star in the galaxies is.
that is the plural form, the singular form is galaxy.