Astronomers "weigh" heaviest known black hole in our cosmic neighborhood
The enormous mass of M87, 6.6 billion Suns, is the largest ever measured for a black hole using a direct technique.
January 13, 2011
Astronomers led by Karl Gebhardt from The University of Texas at Austin have measured the most massive known black hole in our cosmic neighborhood by combining data from a giant telescope in Hawaii and a smaller telescope in Texas. The result is a mass of 6.6 billion Suns for the black hole in the giant elliptical galaxy M87. This enormous mass is the largest ever measured for a black hole using a direct technique. Given its massive size, M87 is the best candidate for future studies to actually "see" a black hole for the first time, rather than relying on indirect evidence of their existence as astronomers have for decades.
Artist's concept of what a future telescope might see in looking at the black hole at the heart of the galaxy M87. Clumpy gas swirls around the black hole in an accretion disk, feeding the central beast. The black area at center is the black hole itself, defined by the event horizon, beyond which nothing can escape. The bright blue jet shooting from the region of the black hole is created by gas that never made it into the hole itself but was instead funneled into a very energetic jet.
Photo by Gemini Observatory/AURA illustration by Lynette Cook
Gebhardt led a team of researchers using the 8-meter Gemini North telescope in Hawaii to probe the motions of stars around the black hole in the center of the massive galaxy M87.
Jeremy Murphy from the University of Texas used the Harlan J. Smith Telescope at the university's McDonald Observatory in West Texas to probe the outer reaches of M87 — the so-called "dark halo." The dark halo is a region surrounding the galaxy filled with dark matter, an unknown type of mass that gives off no light but is detectable by its gravitational effect on other objects.
In order to pin down the black hole's mass conclusively, Gebhardt said, one must account for all the components in the galaxy. Thus, studies of both the central and outermost regions of a galaxy are necessary to see the influence of the dark halo, the black hole, and the stars. But when all of these components are considered together, Gebhardt said, the results on the black hole are definitive, meeting what he calls the "gold standard" for accurately sizing up a black hole.
Gebhardt used the Near-Infrared Field Spectrograph (NIFS) on Gemini to measure the speed of the stars as they orbit the black hole. The study was improved by Gemini's use of "adaptive optics," a system that compensates, in real time, for shifts in the atmosphere that can blur details seen by telescopes on the ground.
Together with the telescope's large collecting area, the adaptive optics system allowed Gebhardt and Joshua Adams from the University of Texas to track the stars at M87's heart with 10 times greater resolution than previous studies. It's "just an enormous improvement compared to previous work," Adams said.
"Our ability to obtain such a robust black hole mass for M87 bodes well for our ongoing efforts to hunt for even larger black holes in galaxies more distant than M87," said Tod Lauer from the National Optical Astronomy Observatory.
Texas graduate student Jeremy Murphy used a different instrument to track the motions of stars at the outskirts of the galaxy. Studying the stars' movements in these distant regions gives astronomers insight into what the unseen dark matter in the halo is doing. For this work, Murphy employed an innovative instrument called VIRUS-P on McDonald Observatory's Harlan J. Smith Telescope.
"That has been an enormous struggle for a long time, trying to get what the dark halo is doing at the edge of the galaxy, simply because when you look there, the stellar light is faint," Gebhardt said. "This is where the VIRUS-P data comes in, because it can observe such a huge chunk of sky at once."
This means the instrument can add together the faint light from many dim stars to create one detailed observation. This kind of instrument is called an "integral field unit spectrograph," and VIRUS-P is the world's largest.
"The ability of VIRUS-P to dig deep into the outer halo of M87 and tell us how the stars are moving is impressive," Murphy said. "It has quickly become the leading instrument for this type of work."
The combined Gemini and McDonald data have allowed the team to pinpoint the mass of M87's black hole at 6.6 billion Suns. But measuring such a massive black hole is only one step toward a greater goal.
"My ultimate goal is to understand how the stars assembled themselves in a galaxy over time," Gebhardt said.
"How do you make a galaxy?” asked Gebhardt. “These two datasets probe such an enormous range, in terms of what the mass is in the galaxy. That's the first step to answering this question. It's very hard to understand how the mass accumulates unless you know exactly what's the distribution of mass — how much is in the black hole, how much is in the stars, how much is in the dark halo."
Today's conclusions also hint at another tantalizing possibility for the future — the chance to actually "see" a black hole.
"There's no direct evidence yet that black holes exist," Gebhardt said. "... Zero, absolutely zero observational evidence. To infer a black hole currently, we choose the ‘none of above’ option. This is basically because alternative explanations are increasingly being ruled out."
However, he says that the black hole in M87 is so massive that astronomers someday may be able to detect its "event horizon" – the edge of a black hole beyond which nothing can escape. The event horizon of M87's black hole is about 3 times larger than the orbit of Pluto — large enough to swallow our solar system.
Though the technology does not yet exist, M87's event horizon covers a patch of sky large enough to be imaged by future telescopes. Gebhardt says future astronomers could use a worldwide network of submillimeter telescopes to look for the shadow of the event horizon on a disk of gas that surrounds M87's black hole.