If it looks like a black hole, and acts like a black hole, it's probably a black hole.
For a while now scientists have thought a dense, massive object lurking at the center of our galaxy is likely a giant black hole, but they haven't been able to prove it. New observations, offering the closest view yet of the heart of the Milky Way, present strong evidence for the black hole theory and raise hopes of finally settling the question soon.
By linking a series of radio telescopes around the world, astronomers created a virtual telescope with the resolving power of a single dish the size of the distance between the various sites (about 2,800 miles, or 4,500 kilometers). This instrument grabbed an intimate image that probed nearly to the Milky Way's black hole's event horizon — the point beyond which nothing, including light, could escape.
Since our own galaxy's apparent supermassive black hole is the closest of its kind to us, it offers a unique chance to study how these objects behave and affect galaxies.
"This is the best black hole candidate that we have anywhere in the universe, the best chance we have to observe the kind of signatures we would expect around the immediate vicinity of a black hole," said study leader Sheperd Doeleman of the Massachusetts Institute of Technology. "One of the problems with looking at this particular source is that we have to look through our galaxy. It's a blessing that it's this close, but it's a curse because it's obscured by gas and dust."
In order to bypass the Milky Way's shroud of gas and dust, the researchers looked at 1.3 mm radio light, which escapes the fog better than longer-wavelength light. They combined observations taken from observatories in Hawaii, Arizona and California in a technique called Very Long Baseline Interferometry to observe the galactic center with some of the highest resolution ever achieved in astronomy —the equivalent of a baseball seen on the surface of the moon, 240,000 miles away.
The researchers observed a bright source of light known as Sagittarius A* ("A-star"), thought to mark a black hole roughly 4 million times the mass of the sun. The mass is determined by looking at the effect of the colossal object on stars that orbit near to the galactic center. The team found that Sagittarius A* has a diameter equal to about one-third the distance between Earth and the sun, or about 30 million miles (50 million kilometers).
This small size indicates the mass in the galactic center is even denser than previous measurements found, which supports the idea that the object hidden there must be a black hole, because current theories have no other reasonable explanation for describing so much mass packed into such a small space.
Settling the question
Scientists can't pin down the process responsible for the bright radiation coming from Sagittarius A*, but suggest it could be a powerful jet of particles accelerated by magnetic fields around the black hole, or radiation pouring out of an accretion disk of matter funneling into the black hole.
The researchers hope they'll be able to get to the bottom of the question, and finally prove that Sagittarius A* is a supermassive black hole, through future observations made with the same technique.
"We've been working for over a decade now on the machinery and instrumentation to pull this off," Doeleman told Space.com. "The real beauty of this technique is that now we've shown it can be done. We'll get very good data in the next three to five years and I think some of those will tell us if we're seeing some of the signatures we'd expect from a black hole."
To improve on the images they've already made, the astronomers plan to add even more telescopes around the world, as well as more dishes at each individual site to boost the signal. They also plan to look in even shorter-wavelength radio light.
"This pioneering paper demonstrates that such observations are feasible," said Harvard astrophysicist Avi Loeb, who did not work on the study. "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."
The study, funded by the National Science Foundation and conducted at 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 the James Clark Maxwell Telescope (JCMT) and the Submillimeter Array (SMA) in Hawaii, is detailed in Thursday's issue of the journal Nature.