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Astronomers Solve Mystery of Formation of First Supermassive Black Holes

Astronomers Solve Mystery of Formation of First Supermassive Black Holes
Gas density distribution around the newborn protostar. The left-to-right supersonic gas motion results in the non-spherical, compressed density structure. (Credit: Shingo Hirano)

An international team of researchers has successfully used a supercomputer simulation to recreate the formation of a massive black hole from supersonic gas streams left over from the Big Bang. The study will be published tomorrow in the journal Science, in a paper led by astronomer Shingo Hirano of The University of Texas at Austin.

"This is significant progress. The origin of the monstrous black holes has been a long-standing mystery and now we have a solution to it," Hirano said.

Recent discoveries of supermassive black holes 13 billion light-years away, corresponding to when the universe was just five percent of its present age, pose a serious challenge to the theory of black hole formation and evolution. The physical mechanisms that form black holes and drive their growth are poorly understood.

Projected density distributions of dark matter (background and top panel) and gas (bottom three panels) components when the massive star forms. The stellar cradle is extremely asymmetrical as a wide, wedge-shaped structure (middle panel) due to the initial supersonic gas motions left over from the Big Bang. The circle in the right panel indicates the gravitationally unstable region with mass of 26,000 solar-masses. (Credit: Shingo Hirano)

Theoretical studies have suggested these black holes formed from remnants of the first generation of stars, or a direct gravitational collapse of a massive primordial gas cloud. However, such theories either have difficulty in forming supermassive black holes fast enough, or resort to very particular conditions.

The research team, led by Principal Investigator Naoki Yoshida of Japan's Kavli Institute for the Physics and Mathematics of the Universe, identified a promising physical process through which a massive black hole could form fast enough. The key was the supersonic gas motions with respect to dark matter. The team's supercomputer simulations showed a massive clump of dark matter had formed when the universe was 100 million years old. Supersonic gas streams generated by the Big Bang were caught by dark matter to form a dense, turbulent gas cloud. Inside, a protostar started to form, and because the surrounding gas provided more than enough material for it to feed on, the star was able to grow extremely big in a short amount of time without releasing a lot of radiation.

"Once reaching the mass of 34,000 times that of our Sun, the star collapsed by its own gravity, leaving a massive black hole," Yoshida said. "These massive black holes born in the early universe continued to grow and merge together to become a supermassive black hole."

"The number density of massive black holes is derived to be approximately one per a volume of three billion light-years on a side — remarkably close to the observed number density of supermassive black holes," Hirano added.

The result from this study will be important for future research into the growth of massive black holes. Especially with the increased number of black hole observations in the far universe that are expected to be made when NASA's James Webb Space Telescope is launched next year.
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Sunday, 22 October 2017

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