New Virtual Telescope Zooms in on Milky Way's Super-Massive Black Hole
Arizona Radio Observatory's SMT is part of the 'virtual' telescope powerful enough to see a baseball on the surface of the moon.

By Lori Stiles, UA News
Sept. 3, 2008

Astronomers have used a University of Arizona radio telescope, along with telescopes in Hawaii and California, to obtain the closest views ever of what is believed to be a super-massive black hole at the center of the Milky Way.

The key to making these observations is a technique called very long baseline interferometry, or VLBI, which links simultaneous observations from several radio telescopes that can be thousands of miles apart. The signals from these radio dishes are combined to create a “virtual” telescope with the same resolving power as a single telescope as large as the distance between the participating dishes. As a result, VLBI can reveal exquisitely sharp details.

The observations would not have been possible without the Arizona Radio Observatory's 10-meter Submillimeter Telescope, or SMT, on Mount Graham, Ariz.

Measurements made between the SMT and the James Clerk Maxwell Telescope in Hawaii provided the critical baseline that established the size of the blackhole, when referenced to measurements between the SMT and the Combined Array for Research in Millimeter-wave Astronomy in eastern California, Arizona Radio Observatory Director Lucy Ziurys of the UA's Steward Observatory said.

"The SMT on Mount Graham played a key role in creating the 'virtual' telescope needed to obtain this result," Ziurys said. She is a co-author of the study published in the Sept. 4 issue of the journal Nature. "The measurements between the SMT in Arizona and the telescopes in Hawaii and California respectively, were absolutely critical to this discovery. They would not have been possible without the sensitive instrumentation and the excellent site quality of the SMT."

The virtual telescope is more than 2,800 miles across and capable of seeing details more than 1,000 times finer than those seen by the Hubble Space Telescope.

The cosmic target of the observations was the source known as Sagittarius A*, or Sagittarius A-star, long thought to mark the position of a black hole whose mass is 4 million times that of the sun. Though Sagittarius A* was discovered three decades ago, the new observations for the first time have an angular resolution, or ability to observe small details, that is matched to the size of the black hole “event horizon” – the region inside of which nothing, including light, can ever escape.

The concept of black holes, objects so dense that their gravitational pull prevents anything including light itself from ever escaping their grasp, has long been hypothesized, but their existence has not yet been proved conclusively.

Astronomers study black holes by detecting the light emitted by matter that heats up as it is pulled closer to the event horizon. By measuring the size of this glowing region at the Milky Way center, the new observations have revealed the highest density yet for the concentration of matter at the center of our galaxy, which “is important new evidence supporting the existence of black holes,” said Sheperd Doeleman of MIT, lead author of the study.

“This technique gives us an unmatched view of the region near the Milky Way’s central black hole,” Doeleman said. “The new observations have a resolution equivalent to being able to see, from Earth, a baseball on the surface of the moon.”

The team developed and installed special equipment at each of the observatories to create the continent-sized telescope.

The astronomers made their observations at 1.3 millimeters, or very short radio wavelengths. These wavelengths can penetrate the fog of interstellar gas that blurs observations at longer wavelengths. Like a distant light seen through a dense mist, longer-wavelength views of the Galactic Center are dimmed and distorted. “The short wavelength observations combined with the large distances between the radio observatories is what makes this virtual telescope uniquely suited to study the black hole," Ziurys said.

Though it takes light more than 25,000 years to reach us from the center of the Milky Way, the team measured the size of Sagittarius A* to be only one-third the Earth-sun distance – a trip that light would make in only three minutes. The astronomers concluded that the source of the radiation likely originates in either a disk of matter swirling in toward the black hole, or a high-speed jet of matter being ejected by the black hole.

This research involved 28 co-authors from several institutions, including the MIT Haystack Observatory, the Harvard-Smithsonian Center for Astrophysics, the Combined Array for Research in Millimeter-wave Astronomy, the UA's Arizona Radio Observatory, the James Clerk Maxwell Telescope, University of California at Berkeley, the California Institute of Technology and the Max Planck Institute for Radioastronomy, among others. This work was funded by the National Science Foundation.


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Lucy Ziurys

Arizona Radio Observatory



Teresa Herbert

MIT News Office