How Do Giant Black Holes Grow? Scientists Stumped

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 released August 2, 2012.

(Image credit: 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.

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.

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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]

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.

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.

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.

Stellar-mass black holes are often paired with normal stars in systems known as X-ray binaries. In the past decade, X-ray observatories have helped scientists learn much about how stellar-mass black holes pull matter from their partner stars. In turn, these findings are helping astrophysicists understand how massive black holes do the same thing, said study co-author Rob Fender at the University of Southampton in England.

X-ray binaries apparently can emit bright outbursts of radiation lasting months to years. Researchers suspect the temperature of the disk of matter accreting onto black holes rises as its mass grows, eventually ionizing the hydrogen within. This increases the viscosity of material in the accretion disk, making it easier for it to slow down and get dragged into the black hole. This increased rush of matter into the black hole is apparently what causes the outbursts of radiation from these systems. Eventually the flow of matter into the black hole slows as the accretion disk runs out of material, which makes the disk cool down, resetting the cycle.

An accretion disk was recently detected swirling tightly around a massive black hole — apparently the remnant of a star that wandered too close. Intriguingly, this material seems to be very near the black hole's event horizon, the boundary beyond which nothing escapes. "By analyzing signals from this material, we can test the predictions of Einstein's theory of general relativity, which is quite exciting," said Rubens Reis, an astrophysicist at the University of Michigan, who authored another study in the same issue of Science.

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Charles Q. Choi is a contributing writer for Live Science and Space.com. He covers all things human origins and astronomy as well as physics, animals and general science topics. Charles has a Master of Arts degree from the University of Missouri-Columbia, School of Journalism and a Bachelor of Arts degree from the University of South Florida. Charles has visited every continent on Earth, drinking rancid yak butter tea in Lhasa, snorkeling with sea lions in the Galapagos and even climbing an iceberg in Antarctica.