Astronomers 'weigh' heaviest black hole yet

Posted by Emma Woollacott

Astronomers have measured the most massive known black hole in our cosmic neighborhood, weighing in at the equivalent of 6.6 billion suns.

Given its hefty proportions, the black hole, in the giant elliptical galaxy M87, could become the first black hole to be directly 'seen', rather than deduced from indirect evidence.

Karl Gebhardt of the University of Texas at Austin led a team of researchers using the eight-meter Gemini North telescope in Hawaii to examine the motions of stars around the black hole.

To pin down the black hole's mass conclusively, Gebhardt says, th team needed to account for all the components in the galaxy, including the dark halo - a region surrounding the galaxy filled with dark matter - the black hole and the stars.

Gebhardt used the Near-Infrared Field Spectrograph on Gemini to measure the speed of the stars as they orbit the black hole. Adaptive optics was used to compensate, in real time, for shifts in the atmosphere that can blur details seen by telescopes on the ground.

The team was able to track the stars at M87's heart with 10 times greater resolution than ever before.

"The result was only possible by combining the advantages of telescope size and spatial resolution at levels usually restricted to ground and space facilities, respectively," Adams says.

The discovery could also lead to the chance to actually 'see' a black hole.

"There's no direct evidence yet that black holes exist," Gebhardt says, "zero, absolutely zero observational evidence. To infer a black hole currently, we choose the 'none of above' option."

He says the black hole in M87 is so massive that astronomers some day could be able to detect its 'event horizon' — its outer edge, beyond which nothing can escape. The event horizon of M87's black hole is about three times larger than the orbit of Pluto — large enough to swallow our solar system whole.

Though the technology doesn't yet exist, Gebhardt says future astronomers could use a world-wide network of submillimeter telescopes to look for the shadow of the event horizon on a disc of gas that surrounds M87's black hole.