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A Quest for Cosmic Karma

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Science  31 Jul 2009:
Vol. 325, Issue 5940, pp. 532-533
DOI: 10.1126/science.325_532

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In an active galactic nucleus (AGN), a black hole shoots out two opposing jets of particles at nearly the speed of light. The shock from the jets drives gas away from the black hole, forming gigantic bubbles that send sound waves rippling through the galaxy's surroundings for millions of years. The process heats the gas farther out, preventing it from cooling and collapsing to form new stars. It also stops more gas from falling into the black hole, which stops gaining mass and gravitational muscle. Both its growth and the growth of the galaxy come to a halt. Astronomers first glimpsed this cosmic shutoff valve in large galaxies in 2003. Since then, such "AGN feedback" has become one of the hottest topics in astrophysics. If it works the way some researchers think it does, AGN feedback is pivotal to understanding how galaxies evolve. It also appears to answer in one stroke two fundamental questions that have plagued researchers: Why are the masses of most galaxies so tightly connected to the masses of the enormous black holes at their centers? And why are very large and very small galaxies so much rarer than theory says they should be?