Lighter than thought. The radio image of this galaxy from 870 million years after the big bang allowed researchers to determine its mass.


The Pit in a Galaxy's Peach

LONG BEACH, CALIFORNIA--Which came first, the black hole or the galaxy? Because most galaxies harbor massive black holes at their centers, astronomers have wondered whether the cosmic beasts were the seeds around which galaxies grew. But black holes are also highly destructive, so they should have kept galaxies from growing. The verdict, presented here yesterday and described at a press conference today at the annual meeting of the American Astronomical Society, is that the black holes came first. Experts say the findings will lead to a better understanding of how galaxies are born.

In recent years, astronomers have learned that galaxies in the nearby universe are uniformly about 700 times as massive as the black holes at their cores. Many think this ratio can provide a clue to the link between the formation of galaxies and black holes. As a first step, a team led by Chris Carilli of the National Radio Astronomy Observatory in Socorro, New Mexico, and Dominik Riechers of the California Institute of Technology in Pasadena set about trying to establish whether black holes lead galaxy growth or vice versa.

The researchers looked at galaxies in the distant universe, peering as far back in time as 1 billion years after the big bang. The only way to study these galaxies in detail is through radio waves, because the black holes at their center--still sucking matter and emanating energy--are too bright for the surrounding galaxy to be visible. Carilli and his colleagues focused on four such objects using the U.S. National Science Foundation's Very Large Array radio telescope in New Mexico and the Plateau de Bure Interferometer in France.

From radio signals associated with the motion of gas molecules within these regions, the researchers deduced the mass of these early galaxies and compared it with the mass of their black holes, which had been obtained from optical data. They found that the galaxies were about 20 to 30 times less massive than expected and only about 30 times as massive as the black holes at their core. If galaxies in the early universe had less matter around their cores than galaxies do today, Carilli and his colleagues reasoned, the cores--the black holes--must have formed before the galaxies did.

"It appears that the black holes come first and grow the galaxy around them," says Carilli. Eventually, like a ripening fruit, the galactic mass gets bigger until its ratio to the black hole mass becomes what astronomers have observed in nearby galaxies. Carilli and his colleagues acknowledge that the results seem to contradict findings suggesting that the enormous energy emanated by active black holes inhibits galaxy growth around them.

Andrew Fabian, an astronomer at the University of Cambridge in the U.K., says the findings are "bound to generate theoretical work" on how black holes and galaxies influence each other. But he's reserving judgment on whether black holes did indeed come first. He notes, for example, that the conclusions are "deduced from gas motions, not stars," which makes them potentially problematic because nongravitational forces such as magnetic fields could cause researchers to underestimate the mass.