Astronomers Solve a 50-Year-Old Mystery About the Milky Way’s Black Hole

At the heart of our galaxy lies a gigantic black hole more than a trillion times heavier than Earth, with all that mass packed into a region about 2,000 times larger than our planet. Scientists discovered that the giant emitted a warm breeze.

The results, detailed today in the Letters from astrophysical journals, suggest not only that all black holes emit such wind, but also that these beasts are not complete loners, isolated from their environment.

“We’ve never seen a breeze coming from a black hole,” says study co-author Elena Murchikova of Northwestern University. “We usually see the aftermath of explosions or other violent activities. Seeing the black hole sitting there, silent but still dumping energy throughout the region without doing anything violent, is awfully cute,” adds Murchikova, an assistant professor in Northwestern’s department of physics and astronomy.

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Supermassive black holes are suspected to lurk at the centers of all galaxies. Despite numerous investigations into our galaxy’s monstrous resident, called Sagittarius A*, or Sgr A* for short, scientists have yet to detect gaseous winds blowing from it, which they have long theorized existed.

“To observe our own black hole, we need to look across the plane of our galaxy,” Murchikova said in a statement. “That means we have to look through gas and dust and ionized structures, and you can’t really see through all of that easily.”

Murchikova and Mark Gorski of Northwestern led a team that compiled five years of data captured by a radio telescope in Chile called the Atacama Large Millimeter/submillimeter Array (ALMA) to create an image of the cold molecular gas surrounding Sgr A*. Once the researchers removed the bright radio light around the black hole from the resulting image, they were able to see previously invisible structures inside the gas. The most striking structure was a cone-shaped cavity about three light years long.

They also found this same cone-shaped void in data collected by NASA’s Chandra X-ray Observatory.

Here’s how the wind likely forms: As the gas gets close enough to feel the gravity of Sagittarius A*, the material begins to heat up and orbit the cosmic juggernaut; the closer it gets to Sgr A*, the faster this material orbits until it is spinning at close to the speed of light. The wildly swirling matter is now trapped by the black hole’s gravitational grip, forming a flattened accretion disk that will soon become Sgr A*’s dinner.

But not everything is sucked up. “Near black holes, the gas is under strong radiation pressure – coming from the same gas but… even closer to the black hole – and various flares can also occur there,” says Murchikova. This pressure projects part of the hot gas outwards in the form of a wind. “In fact, more gas is ejected than falls into the black hole,” she says. When a strong magnetic field is present, this large cone narrows and is called a jet.

The team is excited about this discovery, which could lead to a deeper understanding of these strange objects. “It always seemed a bit peculiar to me that the black hole in the sky that we have the most problems with and that most closely matches our theories is our closest supermassive black hole and the one for which we have the most data,” says Murchikova. This new discovery at least helps explain one mystery, she adds. “The lack of wind was one of the most uncomfortable facts.”