Closing In on the Milky Way's Central Black Hole

New studies track the motion of stars to pin down what holds sway at the heart of our galaxy

ESO/S. Gillessen et al.

A pair of new long-term studies tracking the orbits of stars at the center of the Milky Way Galaxy further refine the evidence that a supermassive black hole lurks there. The two teams of researchers used data going back to 1992 and were even able to track a full revolution of one star, known as S2, around the theorized black hole, known as Sagittarius A*, some four million times the mass of the sun.

Both papers provide new, closely related estimates for the mass of the suspected black hole and the distance between our sun and the galactic center, roughly 26,000 light-years away. These two figures "are fundamental parameters for the Milky Way," says astrophysicist Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, lead author of one of the studies. 

The other study's first author, Andrea Ghez, a professor of astronomy at the University of California, Los Angeles, agrees. "Much of our knowledge of the structure of the Milky Way is tied to...the distance between the sun and the center of our galaxy," she says. To sharpen that knowledge as much as possible, she adds, "you'd really like that number nailed down very precisely."

The study by Ghez and her team appears in the current issue of The Astrophysical Journal; Gillessen and his colleagues are set to publish their findings in an upcoming issue of the same journal.

Direct study of a black hole such as the one widely suspected to exist at the center of our galaxy is tricky, as black holes swallow up nearby light, rendering themselves virtually invisible. But researchers can infer properties of a black hole from its hearty gravitational influence on nearby stars.

Ghez says that tracking the orbits of stars is the most direct way to study the black hole's properties. "The physics is simplest if you can just look at the motions of stars," she says. "Anything else requires much more theory, so you're basically building up a larger house of cards. The beauty of this experiment is that it has very few assumptions."

To limit inherent systematic uncertainties, Ghez's group accounted for overlapping light sources when one star passes in front of another or near the black hole itself, where infalling material emits radiation. The team also had to account for any drift by the bodies in the absence of a fixed reference point, a process that Ghez describes as "more subtle than I could have ever imagined." Gillessen's group took similar steps to identify potential sources of error, refining their coordinate system and examining the effects of image distortion and light-source confusion on the estimates.

In earlier approaches, Ghez says, "we had sort of the teenage view of things. We were very excited, we were very emphatic about what we could do, but a little naive." The new papers, she says, try to adopt a more sophisticated view. "What we learned is that we were less knowledgeable than we thought we were," she says. "So our measurements are better, but we understand that there are systematics, and once you account for them, the measurement of the black hole's properties are, in fact, less precise than we used to think they were."

Ghez's team focused on S2, a relatively bright star with a short orbit around the black hole, whereas Gillessen's group determined the orbits of 28 stars, including S2. "It really is amazing to see that we can describe the motions of that many stars" by assuming one massive central anchor, Gillessen says. "The stars fly around wildly, in all directions, at different radii. But all that governs that is simply Newton's law."

The motion of S2, Gillessen says, gives an outer boundary to the central object, which, combined with its inferred mass, helps prove that it is a black hole. "Having four million solar masses sitting there, not shining...and being confined by [the orbit of] the star S2 is really a convincing case," he says. In 2002, Gillessen says, S2 passed within 16 light-hours of the black hole's event horizon, or point of no return; two years earlier, another star passed even closer, around 11 light-hours.

Sheperd Doeleman, an astrophysicist at the Massachusetts Institute of Technology's Haystack Observatory in Westford, Mass., says that pinning down the black hole's parameters is important work and notes that both groups analyzed mounds of data "with particular attention paid to careful error analysis." At the same time, he says, the studies refine rather than redefine prior understanding of the nature of the galactic center.

With more advanced telescopes and optical techniques, Ghez says, these estimates will continue to evolve. A more finely honed picture of the center of the galaxy could even provide a test bed for the fundamental assumptions of Einsteinian physics.

"In principle, these stars could test general relativity, because they get into a very strong gravitational field at the central black hole," Ghez says. "And if the measurements are precise enough, you should be able to see the impacts on the orbit."

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