The first large black holes in the universe likely formed and
grew deep inside gigantic, starlike cocoons that smothered their
powerful x-ray radiation and prevented surrounding gases from being
blown away, says a new study led by the University of Colorado at
Boulder.
The formation process involved two stages, said Mitchell
Begelman, a professor and the chair of CU-Boulder's astrophysical
and planetary sciences department. The predecessors to black hole
formation, objects called supermassive stars, probably started
forming within the first few hundred million years after the Big
Bang some 14 billion years ago. A supermassive star eventually
would have grown to a huge size -- as much as tens of millions of
times the mass of our sun -- and would have been short-lived, with
its core collapsing in just in few million years, he said.
In the new study to be published in Monthly Notices of the
Royal Astronomical Society in London, Begelman calculated how
supermassive stars might have formed, as well as the masses of
their cores. These calculations allowed him to estimate their
subsequent size and evolution, including how they ultimately left
behind "seed" black holes.
Begelman said the hydrogen-burning supermassive stars would had
to have been stabilized by their own rotation or some other form of
energy like magnetic fields or turbulence in order to facilitate
the speedy growth of black holes at their centers. "What's new here
is we think we have found a new mechanism to form these giant
supermassive stars, which gives us a new way of understanding how
big black holes may have formed relatively fast," said
Begelman.
The main requirement for the formation of supermassive stars is
the accumulation of matter at a rate of about one solar mass per
year, said Begelman. Because of the tremendous amount of matter
consumed by supermassive stars, subsequent seed black holes that
formed in their centers may have started out much bigger than
ordinary black holes -- which are the mass of only a few Earth suns
-- and subsequently grew much faster.
After the seed black holes formed, the process entered its
second stage, which Begelman has dubbed the "quasistar" stage. In
this phase, black holes grew rapidly by swallowing matter from the
bloated envelope of gas surrounding them, which eventually inflated
to a size as large as Earth's solar system and cooled at the same
time, he said.
Once quasistars cooled past a certain point, radiation began
escaping at such a high rate that it caused the gas envelope to
disperse and left behind black holes up to 10,000 times or more the
mass of Earth's sun, Begelman said. With such a big head start over
ordinary black holes, they could have grown into supermassive black
holes millions or billions of times the mass of the sun either by
gobbling up gas from surrounding galaxies or merging with other
black holes in extremely violent galactic collisions.
The quasistar phase was analyzed in a 2008 paper published by
Begelman in collaboration with CU Professor Phil Armitage and
Research Associate Elena Rossi.
"Until recently, the thinking by many has been that supermassive
black holes got their start from the merging of numerous, small
black holes in the universe," he said. "This new model of black
hole development indicates a possible alternate route to their
formation."
Black holes are extremely dense celestial objects believed to be
formed by the collapse of stars and which have such a strong
gravitational field that nothing, not even light, can escape. While
black holes are not directly detectable by astronomers, the
movement of stellar matter swirling around them and powerful jets
of gas blasting outward provides evidence for their existence.
Ordinary black holes are thought to be remnants of stars slightly
larger than our sun that used up their fuel and died, he said.
The supermassive black holes created early in the history of the
universe may have gone on to produce the phenomenon of quasars --
the very bright, energetic centers of distant galaxies that can be
a trillion times brighter than our sun. There also is evidence that
a supermassive black hole inhabits the center of every massive
galaxy today, including our own Milky Way, said Begelman.
"Big black holes formed via these supermassive stars could have
had a huge impact on the evolution of the universe, including
galaxy formation," he said. Begelman is collaborating with
University of Michigan astrophysicist Marta Volonteri, comparing
the possible formation of supermassive black holes from
supermassive stars and quasistars versus their creation by the
merging of ordinary black holes left behind by the collapse of the
universe's earliest stars.
Scientists may be able to use NASA's James Webb Space Telescope,
slated for launch in 2013, to look back in time and hunt for the
cocoon-like supermassive stars near the edges of the early
universe, which would shine brightly in the near infrared portion
of the electromagnetic spectrum, said Begelman.
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