First Black Holes May Have Formed in "Cocoons"
Written by Nancy Atkinson
Very likely, the last image that comes to mind when thinking of black holes is that they need to be nurtured, coddled and protected when young. But new research reveals 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.
"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," said Mitchell Begelman, from the University of Colorado-Boulder. "This new model of black hole development indicates a possible alternate route to their formation."
Ordinary black holes are thought to be remnants of stars slightly larger than our sun that used up their fuel and died.
But the first big black holes likely formed from very large stars that formed early in the Universe, probably within the first few hundred million years after the Big Bang. The unique process of these large stars becoming black holes includes the formation of a protective cocoon, made of gas.
"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.
These early supermassive stars 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.
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.
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.
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. 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.
Begelman said big black holes formed from early supermassive stars could have had a huge impact on the evolution of the universe, including galaxy formation, possibly going on to produce quasars — the very bright, energetic centers of distant galaxies that can be a trillion times brighter than our sun.
Begelman's paper will be published in Monthly Notices of the Royal Astronomical Society.
Source: EurekAlert
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November 24th, 2009 at 9:09 pm
The mind boggles at the thought of a star as large as tens of millions of solar masses. How did they form without blowing themselves apart?
November 24th, 2009 at 9:21 pm
Hmmm – an interesting possibility. Guess we'll have to wait for JWST and the next generation of behemoths to get a clearer idea of what star formation looked like in the the very early universe.
November 24th, 2009 at 9:31 pm
There is some reasoning to this. It is rather hard to get something into a black hole. If you miss slightly the particle you sent will just go into an orbit around a black hole. So you need friction with bulk matter which slows it down and causes it to enter the BH by crossing the event horizon. This is the Goldilocks situation, for if you try to shove too much bulk matter with friction towards a BH it heats up and expands back out before crossing the horizon. It is a bit like trying to flush too much down the toilet.
Conditions had to be fairly unique to generate multi-million solar mass black holes in the period around reionization.
LC
November 25th, 2009 at 4:24 am
I heard a talk last year about black holes being fed by self-gravitating disks. This means that the masses of the disks were so high that their influence on the gravitational potential was not negligible any more.
The presented model indicated that in such a setting SMBHs could form within a billion years.
Probably it's a mixture of all the possibilities
November 26th, 2009 at 10:50 am
This scenario might just do the trick in explaining the rapid, early birth of SMBHs that current observations call for.
In what proportion do these objects occur compared to Population III stars? At what redshifts are the objects expected to be found (z=8-15)?
November 26th, 2009 at 4:27 pm
I might be wrong, but there is the prospect for dark matter as well. Dark matter presumably clumped in anhomogeneous distributions. There is some discussion about dark matter induced popIII stars as I understand. So this material might have been drawn into BHs with relatively little friction and heating which can push ordinary matter away.
LC