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Supermassive stars spawn supermassive black holes
KEITH COOPER
ASTRONOMY NOW
Posted: November 26, 2009


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The supermassive black holes that lurk in the hearts of galaxies may have been born inside supermassive stars that acted as enormous cocoons of gas and dust that provided a wealth of material for the black hole to gorge on and grow rapidly.

In a paper published in the Monthly Notices of the Royal Astronomical Society, Professor Mitchell Begelman of the University of Colorado, Boulder, proposes that the biggest black holes in the Universe were spawned by a generation of supermassive stars millions of times more massive than our Sun. These cosmic beasts weren’t normal stars; existing in the very young Universe, they would have accumulated raw matter from their surroundings at a rate of one solar mass per year. Such objects would ordinarily be highly unstable, but Begelman has calculated how the stars’ own rotation would keep them stabilised for a few million years, long enough for the pressure at their centres to cause their core to collapse into a black hole of a few solar masses.

The supermassive black holes that lurk in the hearts of galaxies may have been born inside supermassive stars.

At this point, the baby black holes would begin to feed on the outer layers, or the cocoon, of the supermassive star, and Begelman describes this as the ‘quasistar’ stage. In previous research, Tom Abel and Marcelo Alvarez of the Kavli Institute for Particle Astrophysics and Cosmology in the USA, and John Wise of NASA’s Goddard Space Flight Centre used supercomputers to model the creation of black holes in the supernovae of the very first generation of stars, known as Population III stars (see http://www.astronomynow.com/news/n0908/12bh/). They found that the radiation from these stars, which were the most extreme stars to have existed in the Universe, was enough to blow away any surrounding gas, effectively starving the resulting black hole so that it could not grow. In Begelman’s model, the cocoon of the supermassive star is too thick to evaporate so quickly, allowing the black hole chance to grow. However, past a certain point the temperature of the cocoon would cool enough to become transparent, and radiation from the disc of spiralling gas around the black hole inside would begin to emerge, blowing the cocoon away, leaving a medium-mass black hole some 10,000 times more massive than our Sun.

Although a far cry from the 4.3 million solar mass black hole at the centre of the Milky Way, these black holes would have had a head start over those formed earlier in the scenario described by Abel, Alvarez and Wise. “The Abel–Alavarez–Wise model is still dealing with Population III stars, which form in haloes with temperatures much less than a thousand degrees and where inflow rates are a thousandth of a solar mass per year or less. Under these conditions stellar feedback seems to have a critical regulating effect on accretion into black holes,” Begelman tells Astronomy Now. “In the larger haloes, with temperatures of tens of thousands of degrees and inflow rates of around a solar mass per year, I argue that you can build up a much larger object that is harder to blow apart, and which therefore can create a larger seed black hole.”

In the Abel–Alavarez–Wise model, the radiation feedback from the star heats surrounding gas clouds, meaning that they become too hot to collapse into stars. The hot clouds merge and grow to masses of tens of thousands of times greater than the mass of the Sun, until gravity does finally win out and the cloud collapses to directly form a black hole. Begelman thinks that model still holds, but only that it takes place earlier in history than his supermassive star scenario. The trade-off with his model, he says, is that you get the seeds of supermassive black holes forming later, but they are more massive at birth. He is currently working to find out which scenario is the dominant one for forming supermassive black holes.

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