How does cosmic feedback work and influence galaxy formation?
All large galaxies appear to have a massive black hole at their center. The mass of the black hole is about 0.2% of the mass of the galaxy’s bulge. It is now widely considered that the black hole has brought this about by regulating the amount of gas available for star formation in the galaxy. Massive black holes thereby have a profound influence on the evolution of galaxies, and possibly on their formation. The relative size of a black hole to a galaxy is roughly in the same ratio as a person is to the Earth. Something very small is determining the growth of something very large. This is possible because the gravitational potential well of a black hole is a million times deeper than that of a galaxy (the square of the ratio of the velocity of matter falling into the black hole, 300,000 km/s, to the typical velocity of stars in a galaxy of 300 km/s). As a black hole grows in mass through accreting matter it also releases energy at a level of nearly 100 times the gravitational binding e
