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Mysterious black hole type perplexes, bewilders

Billions of times the mass of our own sun, so-called massive black holes present a baffling mystery with clues hidden at the very dawn of existence.

By Charles Q. / August 3, 2012

Simulation of the formation of a galaxy similar to our Milky Way. Massive black holes lurk in the centers of many of the building blocks that merge to assemble the galaxy, and a black hole also inhabits the final galaxy. Still from video release.

Jillian Bellovary, Fabio Governato


Black holes have long been cosmic mysteries, even as research in the past decade has shed some light on the largest of these dark objects and how matter pours into them.

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Scientists say the many remaining puzzles include how the largest black holes were born at the dawn of the universe and how black holes may help shape the fate of galaxies.

"The whole field of 'massive black holes in galaxies' really developed in its own right in the last 12 years, and now it's one of the hottest in astronomy," Marta Volonteri, an astrophysicist at the Institute of Astrophysics in Paris, told Volonteri is the author of a study on the evolution of the largest black holes appearing in the Aug. 3 issue of the journal Science.

Black holes have gravitational pulls so powerful, nothing can escape, not even light. Astronomers began mulling over the possibility of black holes in 1783, and scientists used Einstein's theory of general relativity to predict black holes in 1916.

Astronomers know of two "flavors" of black holes: "stellar-mass," which are up to a few dozen times the mass of Earth's sun, and "massive," which can be billions of times the sun's mass — nearly the mass of all the stars in the entire Milky Way galaxy. Stellar-mass black holes are known to be the remains of dead stars, but it remains a mystery how the far-more-massive black holes formed. [Photos: Black Holes of the Universe]

The first big black holes

One of the most popular theories about the formation of massive black holes links them with the first generation of stars. However, recent simulations suggest these stars were no more than a few dozen times the sun's mass, Volonteri said – too small to easily achieve the mass of the largest black holes.

One alternative scenario proposes that massive black holes originated from stars up to 1 million times the sun's mass born from gas that rapidly accumulated over less than about 2 million years. Another model suggests the kernels of massive black holes were born from dense clusters of stars in the centers of galaxies that merged to form stars up to a few thousand times the sun's mass, which in turn collapsed to create black holes.

Curiously, astronomers recently discovered that billion-solar-mass black holes existed when the cosmos was less than a billion years old. Scientists are at a loss to satisfyingly explain how such massive black holes could have formed so early in the universe's history.

After a certain point known as the Eddington limit, the energy released by matter rushing toward a black hole should halt the flow feeding that black hole, restricting how large it can grow. Although it is possible that billion-solar-mass black holes could have formed in less than a billion years after the Big Bang, "all odds must be favorable to the black hole, so only lucky black holes can make it," Volonteri said.

Scientists are now exploring whether or not black holes can overcome the Eddington limit and grow at so-called super-Eddington rates. "That would make black hole growth easier," Volonteri said.

Black holes and their galaxies

It remains a hotly debated question whether these massive black holes dominate how their galaxies grow or vice versa. Perhaps the energy these black holes release alters the overall evolution of galactic structures, or perhaps galaxies control how much gas falls into these black holes and thus regulate how large they grow. Another possibility is that massive black holes and their galaxies develop symbiotically.

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