How would two galaxies in collision produce a supermassive black hole or quasar?
A galactic collision can produce a supermassive black hole and ignite a quasar.
Most galaxies have a supermassive black hole at their centres. So, when two galaxies collide it is likely that there is already at least one supermassive black hole.
If there are two supermassive black holes there are three possibilities. The two supermassive black holes may stay with their respective galaxy. It is possible that the two supermassive black holes will merge into one. It is also possible that one of the supermassive black holes will get ejected.
It is also possible that stellar black holes in the two galaxies merge into a single supermassive black hole. This is less likely as it would require hundreds of thousands or more black holes to merge.
During a collision it is likely that a supermassive black hole will acquire enough material for form a substantial accretion disc. This could come from nebulae or stars which get too close the the supermassive black hole and get torn apart.
If enough material keeps falling into the accretion disc it will get superheated by friction and gravitational effects. This will emit huge amounts of energy and becomes a quasar.