An international team of astrophysicists has developed a new model that shows how an elusive type of black hole can be formed in the gas surrounding their supermassive counterparts.
Image shows the interaction between a massive gas giant planet – comparable in mass to Jupiter – and a surrounding protoplanetary disk of gas and dust. New research predicts that intermediate-mass black holes can create gaps in gas disks around supermassive black holes, analogous to the gaps produced by giant planets in disks around stars. The gap provides a signature that might give scientists the first glimpse of this elusive type of black hole (Phil Armitage / University of Colorado)
In a study, published in the journal Monthly Notices of the Royal Astronomical Society (arXiv.org), the team proposes that intermediate-mass black holes – light-swallowing celestial objects with masses ranging from hundreds to many thousands of times the mass of the Sun – can grow in the gas disks around supermassive black holes in the centers of galaxies. The physical mechanism parallels the model astrophysicists use to describe the growth of giant planets in the gas disks surrounding stars.
“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 co-author Saavik Ford, a research associate in the American Museum of Natural History’s Department of Astrophysics and a professor at the 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 birth of an intermediate black hole starts with the death of a star that forms a stellar or low-mass black hole. In order for this ‘seed’ to grow, it must collide with and consume other dead and living stars. But even though there are many billions of stars in large galaxies, there’s an even greater proportion of empty space, making collisions a very rare occurrence.
The researchers’ new model suggests that previous searches for middleweight black holes might have been focused on the wrong birthing ground.
“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, a research associate in the American Museum of Natural History’s Department of Astrophysics and a professor at CUNY’s Borough of Manhattan Community College.
The new mechanism turns attention instead to active galactic nuclei, the piping hot and ultra-bright cores of galaxies that feed supermassive black holes. The gas in this system is the key, causing the stars to slow down and conform to a circularized orbit.
“You can think of the stars as cars traveling on a 10-lane highway,” Prof 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 resulting collisions allow a stellar black hole to swallow stars and grow. The black hole’s size and gravitational pull increase as its mass expands, escalating its chance of further collisions. This phenomenon, called ‘runaway growth,’ can lead to the creation of an intermediate-mass black hole.
_______
Bibliographic information: McKernan et al. 2012. Intermediate mass black holes in AGN disks – I. Production & Growth. Monthly Notices of the Royal Astronomical Society, first published online 10 July 2012; doi: 10.1111/j.1365-2966.2012.21486.x
