Observations in M87 suggest promise of budding telescope network

By Nadia Drake

Web edition: September 27, 2012
Print edition: November 3, 2012; Vol.182 #9 (p. 10)

A network of radio telescopes has produced the most detailed observations yet of a supermassive black hole in one distant galaxy’s churning heart. The observations, reported online September 27 in Science, may help explain how some active galactic nuclei launch powerful plasma jets thousands of light-years into space.

“This is a tremendous technical achievement,” Stanford University astrophysicist Roger Blandford says of the new observations. “It’s a step along the road to an ambitious goal of imaging a black hole in a galactic nucleus.”

The Event Horizon Telescope network, when complete, will focus on the black holes that power galaxies, cosmic engines so extreme that they could test Einstein’s theory of general relativity. In 2009, astronomers aimed the partially complete radio telescope array at M87, located 53.5 million light-years away in the constellation Virgo. The bright, supergiant elliptical galaxy has a black hole weighing more than 6 billion suns churning in its core.

Because it’s so huge, the black hole is pulling in enormous quantities of gas and dust. The extremely hot, opaque material spiraling in obscures the cosmic drain to all but specific radio wavelengths, which can pierce through the maelstrom and see the structures driving the chaos.

M87 also produces two enormous plasma jets, streams of charged particles traveling at nearly the speed of light, that extend more than 5,000 light-years from the black hole’s poles. Roughly 10 percent of active galactic nuclei emit such jets. Until now, scientists hadn’t been able to see the source of these jets and puzzled over their engines: Did the matter spiraling inward power the jets, or were they launched by the black hole itself?

Over 36 hours of observing time in April 2009, researchers pointed four telescopes in California, Arizona and Hawaii at the veiled heart of M87, observing and measuring the base of an enormous jet erupting from the galaxy’s core.

To astronomers’ surprise, the jet base was narrow enough to almost be a physical impossibility — unless the black hole were spinning at roughly 65 percent the speed of light. Such an interpretation points toward an answer to the still-open question of whether a spinning black hole is required to launch such jets.

“They had to make a couple of assumptions. One is that they’re looking at the base of the jet. They probably are,” says astrophysicist Alan Marscher of Boston University. “The observation is very nice. The interpretation needs confirmation.”

M87’s jets are formed from particles that are swept up as they approach the black hole and launched outward before being crunched into nothingness. In these extreme environments, magnetic field lines coming from the matter spiraling inward can act as rubber slingshots. They twist around the particle jet like a helix, squeezing the particle stream, and focusing and accelerating the jet. Although black holes are generally thought of as cosmic drains, such jets help redistribute matter on large scales, and can play a role in the birth and formation of nearby stars and galaxies.

The new study suggests that the observed M87 jet is powered by an area very close to the black hole itself, and not by an engine farther out in the debris disk.

“It doesn’t prove that the power is extracted directly from the black hole, but it’s certainly consistent with that,” says Blandford, who developed early theories describing such black hole–powered jets.

The findings also match observations published this year in Astrophysical Journal Letters which suggest that the M87 jet base is indeed quite narrow. “Their conclusions are quite within our previous expectations,” says Masanori Nakamura, an astrophysicist at Taiwan’s Academia Sinica Institute of Astronomy & Astrophysics, who studies M87.

Because the environment around a behemoth black hole is dominated by extreme gravitational forces, scientists are hoping to use the Event Horizon Telescope to test Einstein’s theory of general relativity. “If Einstein’s theory is going to break down, this is where it’ll happen,” says coauthor Shep Doeleman of MIT's Haystack Observatory in Westford, Mass. The team hopes within the next few years to verify whether spacetime curving around a black hole produces an observable “black hole shadow” — a dark, circular disk surrounded by a ring of photons. So far, Doeleman says, Einstein’s calculations are holding up well. “It’s never a good idea to bet against Einstein.”