Collisions of individual stars would be very unlikely because of the vast spaces between them, but each galaxy can be distorted or disrupted by the gravity of the other. Gravitational disturbances and collisions of giant gas clouds can trigger bursts of star formation. Galaxy collisions can transform spiral galaxies into ellipticals, and at least some elliptical galaxies probably formed this way. The most luminous quasars are 100-1000 times brighter than the whole Milky Way galaxy. Many emit radio waves, visible light, X-rays, and gamma rays.
Some quasars vary in brightness over weeks or even days.
The enormous luminosity must therefore be coming from a fairly small region, no more than a few light-weeks across. We see light from hot gas spiraling in towards the black hole. Luminous quasars in the nearby universe are very rare. Most quasars have died out. The age of quasars is over.