A Close Look at the Black Hole in the Milky Way

An artist’s concept illustrating the new view of the Milky Way, along with other findings presented at the 212th American Astronomical Society meeting in St. Louis, Mo. The galaxy’s two major arms (Scutum-Centaurus and Perseus) can be seen attached to the ends of a thick central bar, while the two now-demoted minor arms (Norma and Sagittarius) are less distinct and located between the major arms. The major arms consist of the highest densities of both young and old stars; the minor arms are primarily filled with gas and pockets of star-forming activity (credit: NASA/JPL-Caltech)

Most galaxies are hosts to Super-Massive Black Holes (SMBHs) lying at their centers. The Milky Way is no different. Every black hole has an event horizon, a region beyond which nothing can emerge, not even light. Therefore direct observations of the black hole itself are currently impossible, and astronomers have to settle for observations of the areas surrounding the black holes.

Such an observation was recently made by an international team of astronomers led by Sheperd Doeleman of MIT. The team was able to detect a structure near the Milky Way’s black hole, at an angular scale of 37 micro arc-seconds. This could be compared to detecting a baseball on the moon. In order to perform this feat researchers had to combine radio wave emissions captured by telescopes in Hawaii, Arizona, and California, using a technique called Very Long Baseline Interferometry (VLBI). “This technique gives us an unmatched view of the region near the Milky Way’s central black hole,” said Sheperd Doeleman. “No one has seen such a fine-grained view of the galactic center before,” agreed Jonathan Weintroub of the Harvard-Smithsonian Center for Astrophysics, another team member. “We’ve observed nearly to the scale of the black hole event horizon.”

The team studied 1.3 mm radio wave emissions from an object named Sagittarius A*. Emissions at this wavelength are more likely to escape the galactic center, because they are not as susceptible to interstellar scattering which results in a blurred image. The VLBI technique is normally limited to wavelengths of 3.5 mm or longer, but the team was able to push the technique’s limits and obtain results for a shorter wavelength of 1.3 mm.

Graphic shows four radio observatories in Hawaii, Arizona and California linked to create a 2,800-mile-wide virtual telescope for detecting black holes. (Credit: Sheperd Doeleman, MIT)

The structure they saw was located at the galactic center and had a 37 micro arc-second angular scale, corresponding to about 30 million miles. They were able to only vaguely determine its shape. Further research is required to understand what this structure is. Options include a glowing corona surrounding the black hole, an orbiting “hot spot” or a material jet.

“This pioneering paper demonstrates that such observations are feasible,” commented theorist Avi Loeb of Harvard University, who is not a member of the discovery 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.” Furthermore, “This result, which is remarkable in and of itself, also confirms that the 1.3-mm VLBI technique has enormous potential, both for probing the galactic center and for studying other phenomena at similar small scales,” said Weintroub.

The team plans to continue its work by developing novel instrumentation to make more sensitive 1.3-mm observations possible. They also hope to develop additional observing stations to enhance the detail in the picture. Future plans also include observations at shorter, 0.85-mm wavelengths. However, for such work to be successful, they will have to stretch the capabilities of the instrumentation even further and wait for excellent weather conditions at all telescope sites.

TFOT reported on research confirming the leading theory regarding the behavior of galactic black holes, according to which the particles are accelerated by tightly-twisted magnetic fields close to the black hole. In another article TFOT covered a new study suggesting there is an upper limit on the mass of black holes.

Further information on the new discovery, which was published in the Sept. 4^th issue of Nature, can be found in the Harvard-Smithsonian Center for Astrophysics press release.