(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.
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"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.
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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