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Black holes got off to
a false start


Posted: August 12, 2009

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It took a generation of starved black holes to spur into action the formation of the giant supermassive black holes that we observe in the centres of galaxies today.

The gas that makes up galaxies has been around since the first few hundred thousand years after the big bang, but the stars and black holes came later. Indeed, the first stars didn’t show up on the scene until 400 million years after the big bang, each one hundreds of times the mass of our Sun and embedded in an enormous halo of dark matter. The question is, how did the Universe get from this first generation of stars to the massive, active black holes seen in galaxies a billion years after the big bang?

A still from the simulations showing gas (in blue) interacting with one of the first black holes (in white). Image: Marcelo Alvarez/John H Wise/Tom Abel.

These first stars only lasted a few million years before running out of energy and collapsing upon themselves to form black holes. It had been assumed that it was these black holes that then grew rapidly to form the supermassive black holes, millions of times the mass of our Sun, at the centre of galaxies in the dark matter haloes.

However, new supercomputer calculations that chronicle the first few hundred million years of history have told a different story. Marcelo Alvarez and Tom Abel of the Kavli Institute for Particle Astrophysics and Cosmology, and John Wise of NASA’s Goddard Space Flight Centre, found that in the simulations the first black holes grew much too slowly, at a rate of only a few percent of of their original mass over a hundred million years. This is because the stars that had preceded them had ionised the gas out to a distance of a thousand light years, heating it to such an extent that the star’s radiation began to push the gas away. After the black hole formed, this gas fell slowly back towards it, but even the merest wisps of gas swallowed up by the black holes would generate enough X-ray radiation to heat the surrounding gas further and prevent much more from falling in. In essence the black holes were starving themselves, and were unable to grow into black holes millions of times more massive around which galaxies could form.

So the Universe instead put plan B into operation. “While X-rays from matter falling into the first black holes hindered their further growth, that very same radiation may have later cleared the way for direct formation of supermassive black holes by suppressing star formation,” says Alvarez.

Gas clouds can only collapse to form stars when they are cool, just ten or twenty degrees above absolute zero. The hotter the gas is, the greater the Jeans mass has to be – this is the minimum mass for a gas cloud of a given temperature and density to collapse into a star. The X-ray heating meant that the clouds were able to grow much larger than is typical today without forming any stars. Eventually, however, gravity won the battle, and the entirety of these clouds would collapse in on themselves, so massive that they formed not stars, but supermassive black holes. These black holes would then continue to produce X-rays, but Abel speculates that if surrounding gas clouds are dense enough, they may shield star-forming regions in their shadows.

“This work will likely make people rethink how the radiation from these black holes affected the surrounding environment,” says Wise.

The research, published in Astrophysical Journal Letters, is only the beginning, warns Alvarez. “A lot of work remains to be done to test whether this idea will actually pan out; this is really just the tip of the iceberg in terms of realistic simulations of black holes in the early Universe.”