In The Field

APS 2010: The hair on a black hole

smbh.gif Have you ever seen the videos of stars whirling around the supermassive black hole at the center of the Milky Way? They’re wild. But wouldn’t you want to see pictures of the black hole itself?

On Saturday at APS, there were talks showing how two rival approaches are getting up close and personal to the hole, called Sagittarius A*. One group even says that they will soon be able to actually image the hole — and that the data they have already shows that the hole’s accretion disk is oblique to use, rather than facing us perpendicularly, like a doughnut.

By analyzing the orbits of those nearby stars, astronomers already know the mass of Sgr A* quite precisely — it has swallowed the equivalent mass of four million Suns. But the second fundamental characteristic in defining a black hole is its spin, which would explain something about the history of the hole — a sort of timeline of its appetite — that in turn would help explain the history of the galaxy. And David Merritt of RIT said that scientists are close to spotting the spin. He just needs to find two stars within 20 astronomical units (an AU, or the distance between the Earth and Sun) of the hole and track them. And he says a new instrument, called GRAVITY, expected to be put onto ESO’s VLT in a few years, should do the trick.

While the VLT will peer past the Milky Way’s dust to get at Sgr A* in the near infrared, Shep Doeleman at MIT wants to use radio telescopes to achieve an incredible resolution that he compares to reading the date off a quarter held up in Los Angeles from Washington DC. He has already linked up three radio telescopes — arrays in California, Arizona and Hawaii — giving him an effective baseline of a single telescope 4,500 kilometres across. In a 2008 Nature paper, he used this set up to show that the Sgr A* was one-third of an AU across — which essentially proves that Sgr A* is a black hole, since nothing else could fill up that small of a space with that much mass.

At APS, Doeleman announced that with new data from his radio arrays, he now knows something about the shape of the accretion disk of gas that is feeding the black hole. Instead of facing us like a doughnut, the accretion disk is probably somewhat edge-on or oblique to the Earth. “It’s not as sexy as it used to be,” says Doeleman. Eventually, he wants to bolster his virtual telescope by adding more than a dozen stations in Chile, Antarctica and elsewhere. At that point, this Event Horizon Telescope would be able to make time lapse movies of the hole’s surface, which could be spinning and have hotspots. The animation shown here gives a sense of what some of the researchers think they can achieve.


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