Missing black hole forces galaxy rethink
Posted: Friday, July 20, 2001
(Reuters) Astronomers have found a galaxy that is missing a vital component at its heart - a supermassive black hole. The discovery challenges the prevailing view that these black holes are crucial to galaxy formation.
Supermassive black holes have more than a million times the mass of the Sun and astronomers have found them in every one of the thirty or so galaxies they have examined, usually by observing the gravitational effect they have on stars.
But David Merritt and colleagues at Rutgers University in New Jersey, US, have used the Hubble Space Telescope to image the centre of a disc shaped galaxy called M33. They calculated that the largest black hole it can possibly contain is thousands of times smaller than the smallest supermassive black hole.
Astronomers already knew that M33 was a strange shape - it's a regular disc, without the bulge in the middle that most galaxies display. In general, the smaller the bulge is, the smaller the black hole in the middle of a galaxy. But Merritt's upper limit on the mass of M33's black hole is much smaller than was expected.
Douglas Richstone of the University of Michigan, who has been a prominent champion of the role of black holes in galaxy formation, said he did not understand how bulgeless galaxies like M33 could have formed without a supermassive black hole. "I think it's a problem for the black hole story," he said.
Chicken and egg
The conventional model of how galaxies form is that in the early Universe, gas clouds collapse to form supermassive black holes. Other parts of the gas cloud then swirl into orbit around the black holes and stars are born.
But Merritt says M33 may be the first hard evidence that astronomers have been getting the chicken and egg the wrong way round. "M33 looks like a fairly young galaxy," he says. He thinks it is possible that a supermassive black hole may yet form in M33, although astronomers are not sure whether this would be typical.
Richstone disagrees. Despite the confusion, he is glad that one aspect of the current model survives intact, the relationship between the galactic bulge and the mass of the black hole. He thinks the solution to the problem will come from understanding better how the unusual, bulgeless disc galaxies form.
The limits on the size of M33's black hole are published in Science Express.
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