July 20, 2012
Intermediate Black Holes Need Diet Rich In Stars To Grow
Lawrence LeBlond for redOrbit.com - Your Universe Online
Researchers from several American institutes and agencies have developed a new model showing how an elusive type of black hole can be formed in the gas surrounding their supermassive counterparts.
The scientists, from the American Museum of Natural History (AMNH), the City University of New York (CUNY), the Jet Propulsion Laboratory (JPL) of the California Institute of Technology (Caltech), and the Harvard-Smithsonian Center for Astrophyics, propose that intermediate-mass black holes can grow in the gas disks around supermassive black holes in the centers of galaxies.
“We know about small black holes, which tend to be close to us and have masses a few to 10 times that of our Sun, and we know about supermassive black holes, which are found in the centers of galaxies and have a mass that's millions to billions of times the mass of the sun,” said study coauthor Saavik Ford, research associate at AMNH´s Department of Astrophysics as well as a professor at the Borough of Manhattan Community College, City University of New York (CUNY). “But we have no evidence for the middle stage. Intermediate-mass black holes are much harder to find.”
The researchers know that intermediate black holes must start with the death of a star that forms a stellar or low-mass black hole. In order for this newly-formed black hole to grow, it must collide with and consume other dead and living stars. But even though galaxies are made up of billions of stars, the emptiness of space is far greater, making collisions few and far between.
The new model suggests that previous hunts for intermediate black holes may have been focused on the wrong birthing grounds.
“The recent focus had been on star clusters, but objects there move very quickly and there's no gas, which makes the chances of a collision very slim,” said Barry McKernan, also a research associate at AMNH´s Department of Astrophysics and professor at CUNY's Borough of Manhattan Community College.
The new model focuses instead on active galactic nuclei, the scorching, ultra-bright cores of galaxies that feed on supermassive black holes. The gas in these such systems is key to slowing down stars and conforming them to a circularized orbit.
“You can think of the stars as cars traveling on a 10-lane highway,” McKernan said. “If there were no gas, the cars would be going at very different speeds and mostly staying in their lanes, making the odds of collision low. When you add gas, it slows the cars to matching speeds but also moves them into other lanes, making the odds of collision and consumption much higher.”
The collisions that are borne of result allow a stellar black hole to swallow stars and grow. The black hole´s size and gravitational pull increase as its mass expands, which gives it more chances to collide with and consume further stars. This is known as the “runway growth” and can lead to the creation of an intermediate-mass black hole.
As these black holes grow, they begin to alter the gas disk that controls them. The new model shows that black holes of a certain mass can create a gap in the disk, possibly giving scientists their first glimpse of intermediate black holes.
This model can also describe the mechanism for the formation of gas giants like Jupiter. Much like intermediate black holes, planets like this are believed to have grown in gas disks. With planets, however, they develop within these disks surrounding newly forming stars.
“In some regions, we showed that rocky planets could be moved by the gas into common orbits, where they collide to form objects more than ten times the mass of the Earth, massive enough to attract gas and form gas giant planets,” said Mordecai-Mark Mac Low, chair of the Department of Astrophysics at AMNH. “The creative work described here applies the same principles to the far more massive disks found at the centers of galaxies, to form black holes rather than giant planets.”
The multi-organization work was supported in part by NASA and CUNY.