Even a glutton has limits as to how much food he can eat and so it is with the supermassive black holes at the center of galaxies.

Source: Tiziana Di Matteo, Volker Springel, Lars Hernquist When galaxies collide This composite image shows snapshots from a computer simulation of two galaxies colliding. At top, A, the two spinning galaxies approach each other but don't initially collide; instead, they become bound, with a column of gas connecting them, as their nuclei orbit each other. (Only the gas distribution of these galaxies, not their stars, are shown here.) As depicted in the middle of this image, B, the galaxies eventually collide. The black holes in the nucleus of each galaxy combine and initially gobble up much of the gas. The black holes also energize the gas, transforming the combined galaxy into a quasar. After about 100 million years, however, the power of the black hole blows most of the remaining gas out of the galaxy, as seen in the bottom snapshots, C, and ends the galaxy's quasar phase. Finally, at bottom right, D, the galaxy contains little gas, but a large, supermassive black hole. To see an animated version of this simulation, visit the Web site web.phys.cmu.edu/~tiziana/BHGrow/. Click photo for larger image.

Black holes have a reputation for being voracious -- their gravitational fields are so strong, after all, that not even light can escape their pull -- but astrophysicists are coming to appreciate that they actually may regulate the growth of galaxies, as well as their own.

The difference between a black hole and a glutton, however, is that after a black hole is finished feasting on huge quantities of gas, it doesn't push itself away from the table so much as it heaves the table out of the galaxy.

This critical regulatory role of black holes became clear when scientists at the Max Planck Institute for Astrophysics in Germany developed a computational model for galaxy formation that for the first time incorporated black hole dynamics.

The resulting simulations, published last week in the journal Nature, show that supermassive black holes gorge themselves on gas falling into the galaxy's center and pump enough energy into the surrounding gas to create the super-luminous galactic cores known as quasars. But the simulations also showed that all of that energy eventually blows the remaining gas out of the galaxy -- snuffing out the creation of new stars while also putting the gas out of the black hole's reach.

"We were quite stunned by how much this influences our view of galaxy formation," said Tiziana Di Matteo, who led a research team that included her Max Planck colleague Volker Springel and Harvard University astronomer Lars Hernquist.

The regulatory role of the black hole helps explain why new star formation tends to end abruptly in galaxies, said Di Matteo, who joined Carnegie Mellon University last month as an associate professor of physics. And it helps explain why the size of black holes appear to be proportional to the size and velocity of their host galaxies.

"The general picture, I think, is one a lot of people have been converging toward," said Mitch Begelman, an astronomer at the University of Colorado.

That black holes play some important role in galaxy development has become apparent, Begelman said, as observations have shown that essentially every galaxy has a supermassive black hole at its core and that the mass of that black hole is proportional to the rest of the galaxy. In other words, the larger a galaxy's black hole, the larger the galaxy and the faster its stars move.

But just how black holes were having these effects has not been so obvious, said Avi Loeb, a Harvard University astronomer. Supermassive black holes are, by definition, extremely massive, but they still represent a small fraction of the total mass of a galaxy.

In the case of the Milky Way, for instance, the black hole at the galactic center is about 4 million times as massive as our sun. But the Milky Way as a whole could be almost a million times more massive than that black hole.

And black holes, being very compact, interact with the rest of the galaxies at relatively short distances.

Begelman said a supermassive black hole, which could fit easily into the inner solar system, consumes gases from within a radius of only a light-year or two. That has made it computationally difficult to account for black holes when performing simulations on the scale of a galaxy that might be 100,000 light years in diameter.

Di Matteo and her colleagues focused their simulation on one element of galaxy formation -- the collision of two galaxies, each about the size of the Milky Way.

Though no one is sure how the first galaxies were created -- presumably, gas collects around small black holes -- the collision and merger of small galaxies into larger ones is thought to have been a common phenomenon in the developing universe.

As the galaxies approach each other in the simulation, they both have a lot of energy and so they don't smash into each other, Di Matteo noted. Rather, they sort of graze off each other as they orbit each other and become bound by a band of gas between them. As they lose orbital energy, they eventually come together and the black holes in each galaxy combine into one large black hole.

Initially, this black hole grows as it gobbles up the gas that has been driven into the center of the galaxy by the collision. The galaxy also grows, as some of this gas fragments to become the seeds of new stars.

The black hole not only swallows gas, but energizes the surrounding gas. The gas in the galactic core thus becomes so bright it outshines the rest of the galaxy by 100 times or more, Di Matteo said, creating a quasar.

But this quasar phase is only temporary -- perhaps 100 million years. The gas becomes so energized that what remains is eventually blown out of the galaxy, shutting off new star formation while also halting the growth of the black hole itself.

"It's a suicidal process," said Loeb, whose work with an Australian colleague, Stuart Wyithe, had previously shown that black hole energy could blow gas out of a galaxy. Several previous papers have made this point, he added, but seeing it incorporated into the simulation "makes it more convincing."

Di Matteo and her colleagues also ran a simulation showing the collision of two galaxies without black holes. "At the end of the simulation, the remnant galaxy looks very different" than the simulation that included black holes. A lot of gas is left over, she noted, and star formation continues.

"But that's not what we're seeing in galaxies that are that old," she added. By contrast, the elliptical galaxies produced by the simulations incorporating black holes are similar to those observed in the sky.

"This isn't the end of the story," said Colorado's Begelman, noting Di Matteo and her colleagues made assumptions in the simulation that will require further study.

The researchers made assumptions about how much energy generated by the black hole as it swallows gas is then transferred to the surrounding gas and that the energy is dispersed uniformly rather than in certain directions. "Perhaps it doesn't matter," Begelman added, but other researchers will be performing their own simulations to see what factors are critical and which ones aren't.

For theorists such as herself, Di Matteo said such computational simulations have become an important tool for testing ideas and obtaining insights into some mighty strange phenomena.

"You have your own universe" in the computer, she said, "and you play with it, back and forth."