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Which came first--galaxies or black holes?

LONG BEACH, CALIF.—One of the oddities of the universe revealed over the past decade is that galaxies and the giant black holes at their hubs fit together as if they were made for each other. This is one of those facts of life that sound obvious at first glance, but get stranger the more you think about them.

A giant black hole is a formidable beast, surely able to bend the surrounding swarm of stars to its will. Yet even a giant black hole is still fairly small by the standards of a galaxy, so the galaxy should pay little heed to the monster within. The monster, for its part, is in direct contact with only a fairly small neighborhood and should be oblivious to what happens in the galaxy at large.

Yet astronomers find that black holes consistently have about 0.1 percent of the mass of their galaxies—or, more precisely, of the portion of their galaxies that has an spheroidal shape (which, for an spheroidal galaxy, is the whole thing and, for a spiral galaxy such as our Milky Way, is only the innermost parts). Some astronomers argue that the black hole mass is related not to the mass of the galaxy per se, but to the velocity of the stars, but it amounts to much the same thing: black holes and their host galaxies are blood brothers.

Did the holes come first and guide the formation of their galaxies, did the galaxies come first and build up holes, or did some common factor sculpt both of them? Because galaxies are the building blocks of the universe, this question lies at the heart of many of the puzzles of cosmic evolution.

And the answer? The holes came first, Christopher Carilli of the National Radio Astronomy Observatory and his colleagues announced today at the American Astronomical Society meeting. They used radio telescopes to study four host galaxies bearing black holes from about 12 billion years ago, when the universe was only about a billion years old. By measuring the velocity of gas clouds, they estimated the mass of each galaxy, and by studying the spectral emission lines emitted by material taking a death plunge into the coresponding hole, they estimated the mass of the hole.

These ancient black holes proved to be proportionately much heavier than those in the present-day universe—about 3 percent of the galaxy’s mass. Because black holes never shrink, the galaxy must have beefed up in order to account for the present mass ratio of 0.1 percent. “Black holes came first and somehow—we don’t know how—grew the galaxy around them,” Carilli says.

One caveat is that the study took in only four galaxies, and abnormally massive ones at that. It remains to be seen whether the trend holds for all galaxies at that early stage of cosmic history. Moreover, the study only ascertains which came first. It says nothing about how the black holes originally formed or how, in an apparent case of tail wags dog, they managed to control the formation of entire galaxies. “This doesn’t really address the origin of that relationship,” Carilli says. “What it addresses is when that relationship was established.”

Indeed, the result presents a brand new mystery. Black holes, being rather destructive, could surely stop a galaxy from forming—say, by blasting out radiation or jets of material. As Andrew Fabian of the University of Cambridge discussed in another presentation at the meeting, the formation of a giant black hole should release enough gravitational energy to blast the entire galaxy apart. Yet in the newly discovered cases, the holes appear to be helping their galaxies to form. Maybe black holes belie their monstrous reputation. They might be the great cosmic gardeners, tending the galaxies around them and making some, like ours, hospitable to life.

Update (January 8): Tod Lauer of the National Optical Astronomy Observatory says he worries that the study may have fallen into a statistical trap. Even if the average galaxy is 1,000 times the mass of its hole, there is still a spread around this average -- some galaxies are smaller or bigger. Yet smaller galaxies are inherently more common than bigger ones, so a black hole of given mass is more likely to be found in a smallish galaxy and hence appear oversized. For this reason, the galaxy-hole pairings seen by Carilli's team might not be representative of the ancient universe, in which case the question of which came first remains unanswered.

Radio telescope image of gas in a young galaxy courtesy of NRAO/AUI/NSF, SDSS

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