Close look at faraway black holes

ASTROPHYSICS

Astronomers have managed to look closer than ever before into the edge of a supermassive black hole in the galaxy, M87. They have determined that the high-speed particle jet shooting from the core of galaxy M87 originates within a region that is just  5.5 times larger than the estimated radius of the black hole. This is the first time scientists have made such a measurement, reports  Rowen Cowen

Astronomers have peered closer than ever before at the edge of a supermassive black hole beyond the Milky Way, and have estimated the closest distance that matter can approach it without getting sucked in. A team of astronomers have linked four radio dishes in California, Arizona and Hawaii, and created a single radio telescope that is powerful enough to study the centre of the galaxy M87. They hypothesise that this galaxy houses a massive black, equivalent to 6.2 billion suns.

Sheperd Doeleman of the MIT Haystack observatory in Westford, Mass., and his colleagues have, with the aid of a telescope, determined that the high-speed particle jet shooting from the core of M87, originates within a region that is just 5.5 times larger than the estimated radius of the black hole. This is the first time scientists have made such a measurement. The team reported its findings in Science, a journal, recently.

Roger Blandford, theoretical astrophysicist at Stanford University in Palo Alto, Calif says, “It’s clearly a triumph that they’ve been able to get this high resolution and probe a region so close to an extragalactic black hole.” Since the new measurement confines the massive central object in M87 to a tiny volume, the body needs to be smaller than the jet’s source region. The evidence for the existence of a supermassive black hole in the galaxy is now almost as compelling as that for our own Milky Way, points out Charles Gammie, theorist at the University of Illinois in Urbana-Champaign.

Beyond the abyss

The team of astronomers went beyond the raw measurements of the jet to infer properties about the spin of the black hole and the rotation of its accretion disk.

Doeleman and his colleagues used a model that assumes that the launch site of the jets in M87 is similar to the smallest stable orbit that matter can have without rapidly plunging into the black hole.

According to Einstein’s theory of gravity, the location of this innermost orbit depends on whether the black hole spins. Accounting for gravitational lensing, which magnifies and stretches the innermost light-emitting region seen by the telescope, the smallest stable orbit for a stationary black hole in M87 would be 7.35 times the radius of the black hole.

This is significantly larger than Doeleman’s measurement and has led the researchers to conclude that the black hole must spin.

Both Blandford and Gammie note that the spin deductions are not as robust as the direct size measurements, because no one knows the correct model for explaining how a supermassive black hole generates jets. Doeleman suggests, “the measurements and inferences are baby steps, but they are baby steps in a regime (of strong gravity) that we haven’t had any access to before now.” He further explains, “Even tiptoeing around Einstein’s backyard is heady stuff.”Doeleman and his colleagues hope to double the resolution of their observation by adding 20 or more radio dishes from ALMA, the giant radio array now being built in Chile’s Atacama Desert by 2015. Doeleman also puts forth that with the aid of ALMA, the telescope network will have the sensitivity to make bona fide images of the region surrounding a supermassive black hole and search for a black hole’s shadow.

The theory predicts a relatively circular shape for the shadow. But if the structure of space-time deviates from Einstein’s prediction, then the shadow would have a different shape. Doeleman muses: “This is probably one of the few places in the universe to actually pose the question, was Einstein right?”