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Which came first, black holes or galaxies?

Astronomers may have solved a cosmic chicken-and-the-egg problem: Which came first — galaxies or the supermassive black holes in their cores?
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Astronomers may have solved a cosmic chicken-and-the-egg problem: Which came first — galaxies or the supermassive black holes in their cores?

For several years now, researchers have known that galaxies and black holes must have co-evolved, with budding galaxies feeding material to a growing black hole while the immense gravity of the black hole generated in its vicinity tremendous radiation that in turn powered star formation. But the scientists hadn't pegged the starting point.

"It looks like black holes came first. The evidence is piling up," said Chris Carilli of the National Radio Astronomy Observatory in New Mexico. Carilli presented his team's findings here today at the 213th meeting of the American Astronomical Society.

Previous studies of nearby galaxies revealed an intriguing link between the masses of the black holes at their centers and the mass of the central "bulge" (a mass of tightly packed stars and gas) in the galaxies: The black hole's mass is always about one one-thousandth the mass of the surrounding bulge.

The ratio is the same for galaxies of all ages and sizes, whether the central black hole is a few million or many billions of times the mass of our sun.

"This constant ratio indicates that the black hole and the bulge affect each others' growth in some sort of interactive relationship," said study team member Dominik Riechers of Caltech. "The big question has been whether one grows before the other or if they grow together, maintaining their mass ratio throughout the entire process."

To help answer this question, Carilli, Riechers and the rest of their team used the Very Large Array radio telescope in New Mexico and the Plateau de Bure Interferometer in France to peer back to near the beginning of the universe, thought to be 13.7 billion years ago, when the first galaxies were forming.

"We finally have been able to measure black-hole and bulge masses in several galaxies seen as they were in the first billion years after the Big Bang, and the evidence suggests that the constant ratio seen nearby may not hold in the early universe," said study team member Fabian Walter of the Max-Planck Institute for Radioastronomy in Germany. "The black holes in these young galaxies are much more massive compared to the bulges than those seen in the nearby universe."

The upshot: "The implication is that the black holes started growing first," Walter said.

The next piece to place in the puzzle will be to figure out exactly how black holes and central bulges affect each others' growth and how the bulges eventually race past the black holes to become more massive.

"We don't know what mechanism is at work here, and why, at some point in the process, the 'standard' ratio between the masses is established," Riechers said.

New telescopes currently in the works, including the Expanded Very Large Array and the Atacama Large Millimeter/Submillimeter Array, will be key tools in solving this mystery, Carilli said.