How Do Supermassive Black Holes Get So Big?

Some galaxies could grow so large by consuming one star from a binary system.

ByPete Spotts, Staff writer, The Christian Science Monitor
April 06, 2012, 5:29 PM

April 8, 2012— -- For years, astronomers have puzzled over the diet needed to bulk up supermassive black holes – powerful gravitational traps that lurk in the centers of galaxies.

Now, scientists have added a new ingredient that they say could account for a substantial portion of a supermassive black hole's heft: stars wrested from binary star systems that wandered too close for their own good.

One star in a binary paring gets ejected from the galaxy at extraordinary speeds. The star left behind orbits even closer to the black hole, joining other stars the black hole has orphaned.

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At some point, the zone in which the orphans orbit reaches its capacity. After that, gravitational interactions between all the orphans virtually guarantee that when a new star enters the zone, another star must vanish into the black hole itself, signaling its end with a brief flash of high-energy radiation.

Since about half of the stars in the galaxy appear as binary pairs, they should be abundant enough to feed the process, the team holds.

The calculations that led to this scenario represent "a proof of concept," notes Scott Kenyon, a researcher at the Harvard-Smithsonian Center for Astrophysics, based in Cambridge, Mass., and a member of the team formally presenting the scenario today in the journal Astrophysical Journal Letters.

Processes operating at the heart of the Milky Way, and by extension other galaxies, aren't well understood, Dr. Kenyon explains. The team hopes its work will help open a window on those processes – in particular, how supermassive black holes grow.

Black holes are objects with gravity so strong that not even light can escape. So-called stellar mass black holes form from the remnants of a massive star that ends its life in an enormous explosion known as a supernova.

But the path from a stellar-mass black hole to a supermassive black hole with the mass of millions or billions of stars is far less clear. A range of possible explanations exists for starting the process – from the direct collapse of a gas cloud to the slow accumulation and merger of stellar-mass black holes at a galaxy's center.

Unfortunately, "they all have problems," Kenyon says.

Enter binary stars, which became candidates as black-hole fodder through a somewhat circuitous route.

In 1988, when the study of supermassive black holes in galaxies was in its infancy, Jack Hills, an astrophysicist at the Los Alamos National Laboratory, proposed that the smoking gun for a supermassive black hole at the center a galaxy would come in the form of stars vaulting from the galactic center at speeds of more than 1 million miles an hour – essentially fast enough to escape from the galaxy. He calculated that the gravitational interaction of a supermassive black hole with a close-in binary star would eject one of the two stars and draw the remaining orphan ever closer.

Seventeen years later, with supermassive black holes already an accepted feature in galaxies, Warren Brown at the Center for Astrophysics reported finding the first so-called hypervelocity star heading out of the Milky Way. Since then, astronomers have cataloged about 20 hypervelocity stars leaving the galaxy.

Researchers began to work back from these stellar speedsters to derive an estimate of the number of orphans left in the black hole's vicinity.

"If you just do a back-of-the-envelope calculation, you begin to realize that's a lot of mass – about the same, if not more than the mass of the black hole itself," says Ben Bromley, an astronomer at the University of Utah and the study's lead author. "A good fraction of the black hole's mass likely came from the captured partners" of binaries over the course of the black hole's lifetime so far.

Why would binaries provide ripe pickings for a central black hole? From the black hole's perspective, two stars orbiting a common center of mass represent a significantly larger object than a single star, Dr. Bromley explains, one easier to disrupt at a a longer distance.

With the black hole in the center of the Milky Way, a binary system must approach within about 93 million miles, or one Astronomical Unit (the average distance between the earth and sun), in order to be disrupted. An individual star would need to approach the black hole to within a Mercury-like distance to before the black hole begins to dismantle it, Bromley says.

Over time, the orphaned stars reach a relatively stable number – until a new star is orphaned and dragged into closer proximity to the black hole. At that point, orbital interactions among all the stars in this inner cluster send one hapless star to be torn apart by the black hole itself.

The flare this final dance triggers – a so-called tidal disruption event – has been spotted in other galaxies. Based on these observations researchers estimate the events take place once every 1,000 to 100,000 years. Meanwhile, the galaxy is producing hypervelocity stars at about the same pace range, the team estimates.

The calculations have their limits, Kenyon cautions. For one thing, he says, the team assumes that binary systems at the center of the galaxy have the right-sized stars at the right separation distances to make it easy for the black hole to part them. That may or may not be the case.

In the end, the feedstock adding heft to a supermassive black hole may come from a number of sources. In some cases, however, the team's model for black-hole growth may represent the dominant process. It could explain how black holes in some of the largest elliptical galaxies, with central black holes of several billion solar masses, can bulk up when the galaxies they inhabit have so little gas to feed on.

The team, which included Center for Astrophysics astronomers Margaret Geller and Dr. Brown, notes that additional tests of their proposed feeding mechanism will come with observations of hypervelocity stars and the galactic interior with a new generation of ground and space-based telescopes. These should be able to detect and characterize stars much fainter than current telescopes can capture.

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