The latest piece of modelling a black hole is said to show a black hole can grow rapidly at the centre of a host galaxy if a nearby galaxy emits enough radiation to switch off its capacity to form stars. See https://phys.org/print408627219.html … the collapse of the galaxy and the formation of a million solar mass black hole toakes100,000 years – a mere blip in cosmic time. A few hundred million years later it will have grown into a billion solar mass supermassive black hole.
In the early universe stars and galaxies formed as molecular hydrogen cooled and deflated a primordial plasma of hydrogen and helium. This would have restricted black holes from growing very big as molecular hydrogen turned gas into stars far enough away to escape the black hole's gravitational pull. Hence the idea of radiation to inhibit this occurring.
At www.dailygalaxy.com/my_weblog/2017/03/-solved-existence-of-supermassive-… … is the same story written with a slightly different slant.