What evidence do we have that supports that quasars are the nuclei of very distant galaxies?
Red shift and variability provide evidence that quasars are at the centre of distant galaxies.
Quasars have a very large red shift. This means that they are typically billions of light years away. It also means that we are seeing them as they were billions of years ago. They are young galaxies formed in the early universe.
Quasars emit vast amounts of radiation. Their energy output is also very variable with periods in the order of days or months. This means that they must be very small objects about the size of our solar system.
Of the many theories about what quasars are, it is now accepted that they are powered by supermassive black holes at the centres of galaxies. The accretion disk of material falling into the black hole gets heated by friction and gravitational effects until they emit vast amounts of radiation.
It is now thought that every galaxy has a supermassive black hole at its centre and that the black hole drives the evolution of the galaxy. a galactic supermassive black hole can only be a quasar when there is sufficient material falling into it to generate the energy. This happens when a galaxy is young or when galaxies collide.