When the bright cosmic beacons known as quasars try to devour too much gas, they spew some of it deep into space in fierce jets. This process kills quasars after short lifetimes while stifling the birth of new stars in their host galaxies, according to a new simulation. The result helps explain a basic mystery about why black holes grow only to a certain size in the cores of galaxies.
Quasars are the universe's most luminous sources of energy. Astrophysicists think they arise from white-hot gas spiraling into large black holes inside distant galaxies. Today, nearly all of these super-massive black holes lie dormant inside galaxies, swallowing only small amounts of gas. A few years ago, astronomers found that super-massive black holes obey a startling rule: Each one has grown just big enough to weigh about 1/500th as much as the surrounding bulge of stars in its galaxy, but no more than that (ScienceNOW, 5 June 2000). Astronomers didn't know why growth should stop at exactly that point.
The new model, published in the 10 February Nature, offers a clue. A team led by astrophysicist Tiziana Di Matteo of Carnegie Mellon University in Pittsburgh, Pennsylvania, used a supercomputer to simulate two galaxies colliding in the early universe. For the first time, the model includes the disruptive effects of black holes at the galaxies' cores. When the galaxies merge, their black holes combine. The new giant hole feeds on gas funneled inward by the collision's shock waves. The hole gains mass for about 100 million years as the hot matter around it blazes as a quasar. As its gravity gets stronger, the hole attracts more and more fuel. Ultimately, the energy raging from the quasar's hot matter drives most of the galaxy's gas far into space. That stunts the black hole's growth and also shuts down the quasar, while robbing the galaxy of its raw material to make new stars, Di Matteo says.
The model's close match to what astronomers observe is "surprising," says astrophysicist Mitchell Begelman of JILA at the University of Colorado in Boulder. Researchers don't know how a quasar converts matter into outbursts of energy, Begelman notes, so the model is a simple estimate of a complex process. "But they have to start somewhere, and this is the first simulation that does the whole thing," he says.