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Scientists size up monster black hole

Lynette Cook / Gemini Observatory / AURA

An 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.

Astronomers say they've come up with the definitive estimate for the mass and size of the biggest black hole in our celestial neighborhood, using a method that can now be applied to even bigger monsters beyond.

It's long been known that the supermassive black hole at the center of the galaxy M87 was a big one, but over the years, there's been some debate over just how big it was. Some of the estimates have ranged down toward a mass equivalent to 3 billion suns. In 2009, however, Karl Gebhardt of the University of Texas' McDonald Observatory took a fresh look at old data and came up with an estimate of 6.4 billion suns.

"That had a large uncertainty," Gebhardt told me. Today, Gebhardt and his colleagues announced a new estimate that's based on high-accuracy observations from the Gemini Observatory in Hawaii as well as the McDonald Observatory. The bottom line: M87's black hole is equal to 6.6 billion solar masses, plus or minus 400 million solar masses.

Not bad for a galaxy that's a mere 50 milllion light-years away.

"It is remarkable to have a galaxy of this size and a black hole of this mass so close to us," Gebhardt told journalists at the American Astronomical Society's winter meeting in Seattle. "It really is in our backyard."

The study has been accepted for publication in the Astrophysical Journal.

How they did it
Gebhardt's team used an instrument on the Gemini telescope known as the Near-Infrared Field Spectrograph to measure the speed of the stars orbiting the black hole. The telescope couldn't spot the individual stars — but thanks to adaptive optics, a technique that compensates for atmospheric distortions, the spectral measurements were fine enough to give the astronomers a sense of the average speed for bunches of stars.

One of Gebhardt's colleagues, graduate student Jeremy Murphy, used a different instrument on McDonald's Harlan J. Smith Telescope to track the motions of stars on the outskirts of M87. The VIRUS-P spectrograph gathered light from a "huge chunk of the sky" and come up with an estimate for the galaxy's total mass, including the mass of the dark matter halo surrounding the visible parts of the galaxy. The total came to 5.7 trillion solar masses, Murphy said.

Karl Gebhardt, an astronomer at the University of Texas at Austin, talks about the VIRUS-P instrument.

All those observations were fed into a computer model, which spit out the black hole mass of 6.6 billion suns, spread across a distance of about 20 billion kilometers. "This is three times the size of Pluto's orbit," said Gebhardt, who went on to note that M87's monster "could swallow our solar system whole."

Gebhardt said his team's efforts demonstrated that it was possible to weigh black holes in distant galaxies by measuring stellar motions with high-resolution instruments. "It gives us the confidence to be able to probe the galaxies that are much farther away," he said.

One such galaxy, 3.5 billion light-years away, is said to contain a black hole that's 18 billion times as massive as our sun. That would make even M87's black hole look like a pipsqueak. But the current estimate is based on indirect observations. Gebhardt would love to use more direct methods to confirm the estimate.

"It gives me fodder for future observing proposals," he told me.

To see a black hole
As any science-fiction fan knows, black holes are strange objects so dense that nothing, not even light, can escape its gravitational pull within a boundary known as the "event horizon." They may result from the catastrophic collapse of a dying star, or they may develop as the supermassive core of an entire galaxy. Astronomers have observed jets and bubbles of radiation that appear to emanate from black holes, and they've even spotted the whirling disks of hot material that surround active black holes. But no one has yet seen the black holes themselves.

"There's no direct evidence yet that black holes exist ... zero, absolutely zero observational evidence," Gebhardt said in a Gemini Observatory news release. "To infer a black hole currently, we choose the 'none of above' option. This is basically because alternative explanations are increasingly being ruled out."

Gebhardt would love to see a black hole, or at least the shadow of its event horizon, and he thinks M87 is the best candidate for that because it's so massive. Our own Milky Way's central black hole is much closer, of course — but it's also much smaller, weighing in at a mere 4 million solar masses. (That's 1 percent of the margin of error for the mass estimate of M87.)

Believe it or not, some astronomers already have a plan to look for the Milky Way's black hole, as part of a project called the Event Horizon Telescope. The plan calls for outfitting the radio telescopes of the Very Long Baseline Interferometer with submillimeter-wave receivers, which just might be able to produce an image of the event horizon when they're linked together.

"It's probably two or three years that we'll need to get there," Gebhardt said. And when the Event Horizon Telescope is ready to go, he hopes that M87 will be on the list of targets. "It is by far our cleanest and best case for a black hole," he said.

Other black hole news from the AAS meeting:


These images show one of the newly discovered black-hole pairs. On the left is an image from the Sloan Digital Sky Survey. On the right is a Keck image that resolves two active galactic nuclei, which are powered by massive black holes.

• Astronomers have discovered 16 close-knit pairs of supermassive black holes in merging galaxies, using imagery from the W.M. Keck Observatory in Hawaii. Caltech astronomer S. George Djorgovski said in a news release that the close pairs "are a missing link between the wide binary systems seen previously and the merging black-hole pairs at even smaller separations that we believe must be there." The 16 pairs were found among an array of 50 targets checked with the Keck 10-meter telescopes, with adaptive optics once again playing a crucial role.

The observations support the view that structures in the universe are assembled "through a hierarchy of mergers," Djorgovski told reporters. He joked that it would have been more interesting if the results contradicted theory, "but unfortunately, our understanding seems to be correct."

• Another team of astronomers observed the "heartbeats" seen in the light from a black hole system, using the space-based Chandra X-ray Observatory and the Rossi X-ray Timing Explorer. The study focused on GRS 1915+105, a binary system in the Milky Way galaxy thought to contain a black hole 14 times more massive than the sun. X-ray pulses emanated from the disk of material surrounding the black hole approximately every 50 seconds. By analyzing variations in the X-ray heartbeat, the astronomers could trace the flow of material into the black hole — and away from it.

"Each heartbeat blasts a new blob of material into the line of sight," Harvard's Joey Neilsen explained. "Ninety-five percent of the material coming in from the outside is actually being blown away by the black hole wind." The matter blown away from the accretion disk every day amounts to a third of the mass of the moon, Neilsen said.

Zoom in on the GRS 1915 black hole, courtesy of the Chandra X-ray Observatory.

More about black holes:


In addition to Gebhardt and Murphy, co-authors of "The Black-Hole Mass in M87 From Gemini/NIFS Adaptive Optics Observations" include Joshua Adams, Douglas Richstone, Tod R. Lauer, S.M. Faber, Kayhan Gultekin and Scott Tremaine. Murphy, Gebhardt and Adams are the co-authors of "Galaxy Kinematics With VIRUS-P: The Dark Matter Halo of M87."

In addition to Djorgovski, co-authors of "Hierarchical Assembly of Supermassive Black Holes: Adaptive Optics Imaging of Double-Peaked [O III] Active Galactic Nuclei" include Hai Fu, Adam D. Myers and Lin Yan. The paper has been submitted to Astrophysical Journal Letters.

In addition to Neilsen, co-authors of "The Physics of Disk Winds, Jets, and X-ray Variability in GRS 1915+105" include Julia Lee and Ron Remillard.

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