Getting closer to the Milky Way’s black hole
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: September 04, 2008

By combining telescopes in Hawaii, Arizona and California, a team of astronomers have stared deep into the heart of the supermassive black hole that is thought to lurk at the centre of our own Milky Way Galaxy.

The radio telescopes (the Arizona Radio Observatory's Submillimeter Telescope (ARO-SMT) of the University of Arizona, the Combined Array for Research in Millimeter-wave Astronomy (CARMA) in California, and both the James Clerk Maxwell Telescope (JCMT) and the Submillimeter Array (SMA) in Hawaii) were linked together to create a virtual telescope more than 4,500 kilometres across, – a technique known as Very Long Baseline Interferometry (VLBI) – capable of seeing details more than 1,000 times finer than the Hubble Space Telescope. The target of the observations was the radio source known as Sagittarius A* (Sgr A*), which, residing in the centre of the Milky Way and boasting a mass four million times that of the Sun, has long been thought to mark the position of a black hole.

"This technique gives us an unmatched view of the region near the Milky Way's central black hole," says Sheperd Doeleman of MIT, first author of the study that is published in today’s issue of the journal Nature.

This image is taken from a computer simulation showing the immediate vicinity of the black hole, with the event horizon depicted as a black sphere. The surrounding disc of gas, represented by white and blue rings, whirls around the black hole. The white column over the pole of the black hole represents a jet of gas being ejected from the vicinity of the black hole at nearly the speed of light. Image: NASA .

VLBI is ordinarily limited to wavelengths of 3.5 millimetres (mm) and longer, however, using innovative instrumentation and analysis techniques, the team was able to tease out the 1.3 mm Sgr A* emission that escapes the galactic centre more easily than emissions at longer wavelengths, which tend to suffer from interstellar scattering. "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," says Lucy Ziurys, Director of the Arizona Radio Observatory.

Although Sgr A* was discovered three decades ago, the new observations are the first to resolve exquisitely small details that are matched to the size of the black hole ‘event horizon’, the region inside of which nothing, including light, can ever escape. "No one has seen such a fine-grained view of the galactic centre before," says co-author Jonathan Weintroub of the Harvard-Smithsonian Centre for Astrophysics (CfA).

The concept of black holes as objects so dense that their gravitational pull prevents anything, including light itself, from ever escaping their grasp has long been hypothesised, but their existence has not yet been proved conclusively. Instead, astronomers study these invisible enigmas by detecting the light emitted by matter that heats up as it is pulled closer to the event horizon.

Even though it takes light more than 25,000 years to reach us from the centre of the Milky Way, the team clearly discerned structure with a 37 micro-arcsecond angular scale (about one-hundred millionth of a degree), which corresponds to a size of about 50 million kilometres at the galactic centre, or the equivalent to about one-third the Earth-Sun distance, a distance that light could travel in just three minutes. The diameter of the Galaxy’s black hole is estimated in the range 12 to 24 million kilometres, but the strong bending of light rays within the black hole’s gravitational field can double the apparent size of the event horizon. Therefore, the new observations are bringing astronomers to the threshold of imaging horizon-scale structure, and have revealed the highest density yet for the concentration of matter at the centre of our Galaxy, providing important new evidence supporting the existence of black holes.

This computer simulation shows what a 'hot spot' of gas orbiting a black hole would look like in an extremely high-resolution image. The black hole's strong gravity distorts the appearance of nearby glowing gas, casting a silhouette. The green lines are a coordinate grid, also distorted by the black hole's gravity. Here, the black hole is viewed from an angle of 30 degrees above the disc plane. Image: Avery Broderick (CITA) & Avi Loeb (CfA) .

The astronomers concluded that the source of the radiation likely originates in either a disc of matter swirling in toward the black hole, or a high-speed jet of matter being ejected by the black hole. But with just three ‘baseline’ observatories, the astronomers could only vaguely determine the shape of the emitting region. Future investigations will help answer the question of what, precisely, they are seeing.

"Future observations that create even larger virtual telescopes will be able to pinpoint exactly what makes Sagittarius A* light up," says Doeleman. "Most galaxies are now thought to have black holes at their centres, but because Sagittarius A* is in our own Galaxy, it is our best chance to observe what's happening at an event horizon."

The team plans to expand their work by developing novel instrumentation to make even more sensitive 1.3 mm observations possible. They also hope to develop additional observing stations, which would provide further baselines to enhance the detail in the picture. Future plans also include observations at shorter, 0.85 mm wavelengths, but such work will face challenges in both the instrumentation required, and the need for a perfect weather conditions at all sites, simultaneously.

"This pioneering paper demonstrates that such observations are feasible," commented theorist Avi Loeb of Harvard University, who is not a member of this research team. "It also opens up a new window for probing the structure of space and time near a black hole and testing Einstein's theory of gravity."

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