Black Holes Tied to Galaxy Growth, Study Says

Stefan Lovgren
for National Geographic News
April 6, 2005
By peering into the deep belly of the universe, scientists have found that massive black holes are growing simultaneously with the galaxies in which they are situated.

Using powerful x-ray technology, astronomers surveyed distant galaxies more than ten billion light-years away. They found that the black hole in the center of each galaxy appears to be growing continuously throughout a burst of star formation.

The observations confirm a theory that the total mass of the stars in a galaxy corresponds to the mass of the black hole. The findings suggest that black holes are pivotal to the formation of galaxies and the structure of the universe.

Scientists have shown that pairs of galaxies—and their black holes—often merge together. At the time of such a merger, tidal forces drive a lot of gas toward the center of the galaxies. This gas feeds the black hole and also is available to create numerous stars.

"Although it is just a tiny fraction of the size of the galaxy, the black hole, in essence, [appears to be] able to control the [growth of the] galaxy," said David Alexander, a research fellow based at Cambridge University in England.

Alexander led the study, which appears tomorrow in the journal Nature.

Supermassive Black Holes

A black hole is a place in space where the gravity is so strong that nothing—not even light—can escape it.

There are two types of black holes. The most common are remnants of massive stars that have collapsed. Most galaxies have millions of such black holes.

The others are known as supermassive black holes, which reside at the center of each galaxy.

Scientists are not sure how these central black holes are created. But over the last decade researchers have found that the total mass of stars in galaxies corresponds directly to the size of their central black holes. This relationship suggests that galaxies and their black holes grow at approximately the same time.

To provide observational support for this theory, astronomers studied galaxies some 10 billion light years away. They used NASA's Chandra X-ray Observatory in Huntsville, Alabama, to find strong x-ray glows produced by hot gas swirling around the black holes.

(Black holes are very hard to locate. The only way to "see" one is to detect emissions from the hot matter that orbits a black hole.)

What the scientists observed happened some ten billion years ago. (The galaxies are so distant that it took their x-rays that long to reach the Earth.) The astronomers found that at that time, the young galaxies and the black holes grew.

"People [once] looked at growing black holes and galaxies independently. Black holes didn't seem important to the formation and evolution of the galaxies," Alexander said.

However, he noted that many studies over the last decade have shown that black holes do, indeed, have an effect on the growth of galaxies.

Colliding Galaxies

The catalyst for the joint growth of galaxies and black holes could be the collision and merger of two galaxies of similar size. The phenomenon has been observed by the Hubble Space Telescope. (See a photo of a "cosmic dust bunny," the aftermath of merged galaxies.)

Tiziana Di Matteo, an astrophysicist at Carnegie Mellon University in Pittsburgh, Pennsylvania, led a recent computer simulation of this phenomenon. The program showed that these mergers drive particles and gas toward the central regions of galaxies. The matter produced stars and provided the fuel that feeds the black hole, enabling it to grow.

"It is very exciting that [Alexander and his colleagues] are able to give us a snapshot of the time in a merger between two galaxies when most of the gas is made available to feed both the central black hole and make most of the stars," Di Matteo said.

As gas cools, the galaxy is created. (Cold gas forms the stars in the galaxy.) At the same time, it only takes a very small amount of gas to be fed into the black hole to make it emit a very large amount of energy.

This energy eventually heats surrounding gas, stopping it from cooling. Stars stop forming, and the galaxy stops growing.

"The black holes [that we studied] are seen precisely at the point where they can have the most influence on the properties of the galaxies they reside in," said Ian Smail, an astrophysicist at Durham University in England and a co-author of the new study.

"Our proposal is that very soon after this time, the increasing energy output from the black holes turns them into quasars," Smail said, referring to distant galaxies with luminous, massive, growing black holes. "In doing so [they] terminate the star formation in these galaxies."

Three billion years from now, we could see similar events taking place closer to home. Our own galaxy, the Milky Way, is on a collision course with the nearby Andromeda galaxy.

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