March 5, 2009

Rare Black Holes Found Orbiting Each Other

Two enormous black holes appear to be orbiting one another at the center of a faraway galaxy, formed by the collision of two separate galaxies, astronomers from the National Optical Astronomy Observatory (NOAO) in Tucson reported Wednesday.

The two supermassive black holes are locked in orbit about 5 billion light years away from Earth, the scientists said.  One light year is the distance light travels in a year, approximately 6 trillion miles.

Data from Apache Point Observatory in New Mexico provided the best evidence to date of two black holes orbiting each other, something known as a binary system, according to NOAO astronomer and former NOAO director, Todd Boroson.

Scientists believe most or all galaxies contain supermassive black holes at their center.  Indeed, our own Milky Way galaxy has a black hole at its center that is roughly 3 million times the mass of the sun.

When galaxies collide and merge, as they do quite often, the black holes at their center may gravitate toward one another due to their mass, entering into orbit as these two seem to be doing.

These orbiting black holes may ultimately merge into an even larger single black hole, Boroson said.

Although scientists believe binary black holes are relatively common, they have so far been elusive.

The signature of a galaxy's black hole is well known, with material falling into a black hole emitting light in narrow wavelength regions forming emission lines that can be seen when the light is dispersed into a spectrum.

These emission lines carry the information about the speed and direction of the black hole and the material falling into it. If two black holes were present, they would orbit each other before merging and would have a characteristic dual signature in their emission lines. The astronomers found precisely this signature.

Boroson and NOAO Astronomer Tod Lauer used data from the Sloan Digital Sky Survey, a 2.5-meter diameter telescope at Apache Point in southern New Mexico to look for this characteristic dual black hole signature among 17,500 quasars discovered by the survey.

Quasars are the most luminous versions of the general class of objects known as active galaxies, which can be a hundred times brighter than our Milky Way galaxy, and powered by the accretion of material into supermassive black holes in their nuclei.

The matter falling into the black hole doesn't go directly in, but orbits around the black hole forming a flat accretion disc, much like the soap scum on water orbiting around an open drain.

More than 100,000 quasars are known, with most being found in the Sloan Digital Sky Survey and at distances that are billions of light-years away.

Boroson and Lauer had to be particularly careful to eliminate the possibility that they were seeing two galaxies, each with its own black hole, superimposed on each other.

"The double set of broad emission lines is pretty conclusive evidence of two black holes," Boroson said.

"If in fact this were a chance superposition, one of the objects must be quite peculiar. One nice thing about this binary black hole system is that we predict that we will see observable velocity changes within a few years at most. We can test our explanation that the binary black hole system is embedded in a galaxy that is itself the result of a merger of two smaller galaxies, each of which contained one of the two black holes."

"You don't see the black holes. You only see the effects of the black holes," Boroson said in an interview with Reuters.

The smaller black hole has a mass 20 million times that of the sun, while the larger one is 50 times bigger, as determined by the their orbital velocities, the scientists said.

"We've discovered that there's a pretty good correlation between the size of a black hole and the size of the galaxy that it's in. This could be a case where a big galaxy ate a smaller galaxy," Boroson added.

The two black holes are located roughly three-tenths of a light year from each other, and take approximately one hundred years to orbit each other, he said.

The finding is published in this week's edition of the journal Nature.


Image Caption: Artist's conception of the binary supermassive black hole system. Each black hole is surrounded by a disk of material gradually spiraling into its grasp, releasing radiation from x-rays to radio waves. The two black holes complete an orbit around their center of mass every 100 years, traveling with a relative velocity of 6000 kilometers per second. Credit: P. Marenfeld and NOAO/AURA/NSF


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