What Fuels Black Hole Growth?
April 3, 2012

What Fuels Black Hole Growth?

"Black holes are very efficient eating machines“¦ They can double their mass in less than a billion years. That may seem long by human standards, but over the history of the Galaxy it's pretty fast,” said Scott Kenyon of the Harvard-Smithsonian Center for Astrophysics (CfA).

New research presented by a joint team from the CfA and the University of Utah showed these super-massive star destroyers can balloon in size by absorbing one-half of a binary star system.  Binary star systems are comprised of two solar bodies that rotate around a shared center of gravity.

The recent black hole study published in the April 2 online edition of The Astrophysical Journal Letters builds on a 2005 CfA study that illustrated how so called ℠hypervelocity´ stars are flung out of the Milky Way´s center by gravitational forces.

When a binary star system approaches Milky Way´s central black hole, gravitational forces ensnare one of the two stars and eject the other. The captured star then circles the black hole until is sucked into its inescapable vortex.

The latest study correlates the growth in size of the black hole with the evidence surrounding how the hypervelocity stars are ejected.

"We put the numbers together for observed hypervelocity stars and other evidence, and found that the rate of binary encounters [with our galaxy's supermassive black hole] would mean most of the mass of the galaxy's black hole came from binary stars," said lead author Benjamin Bromley of the University of Utah. . "We estimated these interactions for supermassive black holes in other galaxies and found that they too can grow to billions of solar masses in this way."

The group looked at binary stars because they act as “a single object much bigger than the size of the individual stars, so it is going to interact with the black hole more efficiently,” he explained. “The binary doesn´t have to get nearly as close for one of the stars to get ripped away and captured.”

Bromley compared the process of a black hole capturing stars from binary pairs to “filling the bathtub.” Once the tub contains water, it goes down the drain in the same way clusters of star are swallowed into the black hole over millions of years.  Bromley said his study shows that the black hole´s “tub” fills at about the same rate it drains.

The study concluded that the Milky Way´s central black hole catapults a hypervelocity star between every 1,000 to 100,000 years. This matches the rate at which black holes measurably tear up and absorb stars. Based on these rates and the estimated age of the universe at slightly over 10 billion years, researchers concluded that the Milky Way´s black hole has devoured over 10 million stars, which more than accounts for its estimated size of 4 million solar masses.

Current telescopic technology is not capable of producing the sample sizes necessary for proving this new theory. To confirm the theory in the future, more powerful telescopes will need to find-- more stars in the cluster near the Milky Way´s super-massive black hole, a certain rate of hypervelocity stars in southern skies, and more observations of stars being shredded in other galaxies.


Image Caption: Artist´s conception of a supermassive black hole (lower left) with its tremendous gravity capturing one star (bluish, center) from a pair of binary stars, while hurling the second star (yellowish, upper right) away at a hypervelocity of more than 1 million mph. The grayish blobs are other stars captured in a cluster near the black hole. They appear distorted because the black hole´s gravity curves spacetime and thus bends the starlight. Photo Credit: Ben Bromley, University of Utah.