What evidence evidence do we have that supports that quasars are the nuclei of very distant galaxies?

Question

What evidence evidence do we have that supports that quasars are the nuclei of very distant galaxies?

Answer 1

The best evidence is that they are small, very far away, and extremely energetic.  They are known to be small because they fluctuate in brightness over a small period of time; only a few hours.  Since no effect can exceed the speed of light, they can be no more than a few light-hours across, which suggests they are the accretion disks of massive black holes.  Their red shifts indicate that they are billions of light years away, making them objects that existed only in the early universe.  To be that far away and as bright as they are, they must be the most energetic objects in the universe, consistent with accretion disks of supermassive black holes.  I believe they have also seen faint galaxy-like structures surrounding some of these, but I'm not sure about that.

Answer 2

Red Shift is also an important indicator of extreme distances.  That is another way to determine how far these quasars are.