What appears to be the central energy-generating system or engine that is producing prodigious amounts of energy in the centers of galaxies, active galaxies and quasars?
1 Answer
It's believed to be a massive black hole, or more precisely the tremendously strong gravitational field of such a black hole.
Explanation:
When things fall to the ground, they are picking up energy from the gravitational field. In the case of Earth the gravitational field is modest but still offers enough energy to break an egg that rolls off your counter or make meteors glow as they arrive from space. With a massive black hole, the energy makes particles so hot that they emit X-rays (not just the visible light of meteors), and if they are not directly captured they could be flung out with tremendous speed and energy to make powerful jets.
When astronomers see this activity and reckon with the energy required to drive it, the astronomers are themselves driven to invoke a massive black hole. Only that seems able to provide enough energy through its gravitational field.
And they find evidence for such black holes from our own galaxy (as well as some others). The Milky Way's central black hole isn't drawing in enough material (now) to produce the spectacular results described above. But we "see" it by noting the orbital motion of nearby stars and other matter, which is so fast a large, compact mass has to be holding things close. See http://hubblesite.org/reference_desk/faq/answer.php.id=64&cat=exotic.
