Windswept path leading from Sagittarius A* discovered after decades of research
After 50 years of searching, astronomers have finally found evidence of a light wind blowing from the supermassive black hole at the center of the Milky Way.
New sightings reveal a cone-shaped path moving away from Sagittarius A* which could only be sculpted by a hot wind, researchers reported June 4 in Letters from astrophysical journals. “We’ve never seen gentle breezes from black holes, but that’s probably what they do most of their lives,” says astrophysicist Lena Murchikova of Northwestern University in Evanston, Illinois. “Now, for the first time, we see this gentle breeze coming from the black hole.”
As black holes consume gas and dust, their cosmic munching heats nearby debris. This heat emits radiation that repels gases and other materials, creating wind. When a black hole is very consuming, these winds can be very powerful, sending matter out of the galaxy. But when a black hole eats little and is in a calm state, as is currently Sagittarius A*astronomers think that the wind would be lighter.
Astronomers have been searching for this theoretical wind since the discovery of Sagittarius A* in the 1970s, when they were just beginning to think that this unidentified object might be a supermassive black hole. But because our view of the galactic center is shrouded in gas, stars and dust, it’s difficult to see what’s going on there.
Over the past 15 years, evidence of X-ray and gamma-ray winds has been found far from the black hole, even extending outside the galaxysuggesting episodes of strong gales in the past. But nothing near the black hole had been confirmed, so astronomers didn’t know if the black hole was still windy.
In the new study, Murchikova and astronomer Mark Gorski, also at Northwestern, accumulated more than 100 hours of observations of Sagittarius A* taken over five years with the Atacama Large Millimeter/Submillimeter Array radio telescopes in Chile. The observations revealed the distribution of cold carbon monoxide, a key indicator of the gas, close to Sagittarius A*.
By taking advantage of a new way of processing data, the team was able to see gas, dust and other materials 100 times fainter than could be seen before, and create an image of the black hole’s neighborhood 80 times sharper. The results showed a cone-shaped space at the black hole that, unlike the surrounding area, is devoid of cold carbon monoxide. The geometry of the vacuum and previous X-ray observations showing hot gas in the same region suggest the culprit must be a hot wind escaping from the black hole, astronomers say.
“The supermassive black holes in the universe live most of their time in a quiet state,” explains Gorski. “SO [these findings] tell us that even though most black holes are pretty quiet, they still have an impact.
Winds from black holes can slow star formation and limit the food available to the growing black hole, or they can help trigger star formation by compressing dust clouds. Understanding the wind coming from a quiet black hole like Sagittarius A* can shed light on how black holes sculpt their galaxies.
“The potential discovery of a wind from Sagittarius A* is indeed a big deal,” says astrophysicist Rebecca Diesing of Columbia University, who was not involved in the new research. “This would demonstrate that our supermassive black hole is not unique, that it produces a wind like that of other galaxies.”
But Diesing would like to see more evidence than just an absence of gas. Future observations could help confirm the wind by probing the speed of gas swept out of the cavity. It’s even possible to see the edges of the void waver over time, as the wind continues to blow.