Black holes: the heart and lungs of a galaxy

It turns out that the supermassive black holes at the center of galaxies like our's pump out a great deal of energy. This has some interesting effects on the evolution of a galaxy:

Black holes generate green energy - Space.com - MSNBC.com:

... The energy that each black hole emits as jets warms the surrounding environment. This prevents gas from cooling and coalescing into billions of new stars, and places an upper limit on how large a galaxy can grow.'In an environmental sense, the black holes are actually preventing galactic sprawl from taking over the neighborhood,' Weaver said."

Gas rushes into the black hole, pulses of energy rush out. To a physician it sounds like the heart and the lungs of a galaxy. Without these massive black holes it sounds like galaxies would be crowded with stars, and presumably much less hospitable to life ...