Published online 7 January 2009 | Nature | doi:10.1038/news.2009.6

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Did black holes form before galaxies?

Astronomers work on universe's chicken-and-egg problem.

galaxyThis radio-frequency picture reveals interstellar gas in a young galaxy dating from when the Universe was only 870 million years old.NRAO/AUI/NSF, SDSS

The huge black holes that lie at the centre of galaxies grow by devouring gas and stars that come too close, but their gravitational attraction can also encourage the birth of stars and the growth of galaxies. This dual role as creator and destroyer has left astronomers with a puzzle: which came first, the black hole or the galaxy?

Research from radio astronomers now suggests that black holes got off to a faster start, at least in four galaxies that existed in the early Universe. "The significant implication is that the black holes formed first and then somehow they formed a stellar galaxy around them," says Chris Carilli, of the National Radio Astronomy Observatory in Socorro, New Mexico, who presented the research at a meeting of the American Astronomical Society in Long Beach, California, on 6 January.

Astronomers had already identified what seemed to be a predictable relationship between the mass of the black hole at a galaxy's heart, and the mass of the galaxy's central bulge of stars and gas. The galactic bulge tends to be 700 times the mass of the black hole, a proportion that holds true for galaxies throughout billions of years of the Universe's history.

But the ratio for the oldest galaxies — formed within the first billion years after the Universe's birth some 13.7 billion years ago — remained a mystery. Quasar black holes at the centre of these galaxies glow so brightly that they prevent optical telescopes from seeing the surrounding galaxies in detail, so that galactic mass estimates were mostly impossible.

Shrinking galaxies

Carilli and his colleagues have now used the Very Large Array radio-telescopes in New Mexico to study four of these galaxies, including one that existed when the Universe was only 870 million years old. The researchers tracked the motion of carbon monoxide gas and used that to estimate the mass of the galaxies — which turned out to be smaller than expected.

The mass of the four galaxies was only 30 times the mass of their black holes, a much smaller ratio than predicted by the normal relationship between black holes and their galaxies. This implies that the black holes were in place — and big — before the galaxies had formed fully.

"No one has really done this in the distant Universe," says Carilli. But he adds that more observations, using the Atacama Large Millimeter/submillimeter Array (ALMA) under construction in Chile, for example, are needed to confirm the finding.

Tod Lauer, an astronomer at the National Optical Astronomy Observatory in Tucson, Arizona, who was unaffiliated with Carilli's work, expressed some skepticism over its statistics. He has found that the variability in black hole size for the biggest, most luminous galaxies is larger. Finding four galaxies with extra-large black holes might be expected by chance alone, he says. "I'm not challenging the observation; I'm challenging the inference from it," he says.

If the observations do hold, astronomers will be faced with a new task: explaining how the black holes came to be so big, so quickly, without a surrounding galaxy for nourishment. They would have to devour gas at maximum rates starting almost immediately after the Big Bang, says Carilli. "It's a total puzzle to me." 

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