Monster Black Holes Spawned Early Galaxies

Ker Than in Long Beach, California
for National Geographic News
January 8, 2009
Monster black holes served as seeds from which early galaxies sprouted, new research suggests.

The discovery could solve the cosmic chicken-and-egg riddle of which came first—galaxies or the supermassive black holes nestled in their cores.

Supermassive black holes have masses equal to a billion suns or more, and they have been detected at the center of many large galaxies, including our own Milky Way.

Previous studies show an intriguing link between the masses of black holes and the central "bulges" of stars and gas in their resident galaxies.

Regardless of their sizes or ages, the bulges of large galaxies appear to be about 700 times more massive than their central black holes. (See black hole photos.)

"We essentially have no clue as to how this relationship [between galaxies and black holes] is established and when," said study leader Chris Carilli of the National Radio Astronomy Observatory (NRAO).

The research, presented at the American Astronomical Society's meeting in Long Beach, California, indicates that galactic bulges and the monster black holes inside them interact with one another and affect each other's growth.

"The big question has been whether one grows before the other, or if they grow together, maintaining their mass ratio throughout the entire process," study team member Dominik Riechers of Caltech said in a statement.

But the research also raises another mystery: How black holes, which have a very strong gravity, can not only hinder but also promote star and galaxy growth.

Growth Spurt

The team used the Very Large Array radio telescope and the Plateau de Bure Interferometer in France to study galaxies formed during the first billion years of the universe.

The findings showed that bulges in four young galaxies harboring quasars—very energetic supermassive black holes—were much smaller compared to their central black holes than later galaxies.

Instead of having 700 times more mass than their supermassive black holes, the bulges in the young quasars were only about 30 times more massive.

One possible explanation for this discrepancy between early and late galaxies is that the supermassive black holes formed first and that galaxies essentially "grew around them," study leader Carilli said.

Another possibility is that the galaxies and their supermassive black holes formed at roughly the same time but grew at different rates throughout their histories.

"Maybe we're just seeing a growth spurt of the black holes," Carilli told National Geographic News.

Another Mystery

Astronomers generally agree that supermassive black holes help slow down stellar formation in their galaxies by disrupting gas clouds with their energetic outbursts, preventing them from coalescing into stars.

But the new findings suggest supermassive black holes must also be able to promote stellar growth and help bulk up their host galaxies.

"There are a lot of very good theories about negative feedback, but how you actually get positive feedback, which is required given these measurements, is much less clear," Carilli said.

(Related: "Black Holes Can Spawn Large, Odd Stars, Model Suggests" [August 22, 2008].)

Karl Gebhardt is an astronomer at the University of Texas in Austin who was not involved in the new research.

Gebhardt agreed that the evidence seems to suggest that, at least in the early universe, supermassive black holes formed before their host galaxies. But whether this is also true for later galaxies is still unclear.

"We just don't know what these galaxies that Carilli and company are looking at will turn into," Gebhardt said. "It's going to take a much larger sample to answer that question."

Mateusz Ruszkowski, a black hole researcher at the University of Michigan, also did not participate in the research.

The new finding implies "that black holes waste no time to begin their growth and don't wait for the bulges to catch up until late in the history of the universe," Ruszkowski said.

"This would have serious consequences for the attempts to model galaxy formation and should help to constrain such models."

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