Black hole wind emerges after 50-year search

(CN) — The supermassive black hole at the center of the Milky Way has long appeared to be missing something astronomers believed every black hole should have.

After more than 50 years of searching, researchers say they have finally found evidence of a wind blowing from Sagittarius A*, resolving a long-standing mystery about the galaxy’s central black hole.

The findings, published Thursday in the journal The Astrophysical Journal Letters, offer a rare glimpse into a relatively quiet phase of a black hole’s life.

Using some of the most detailed observations ever taken of the Milky Way’s core, astrophysicists at Northwestern University uncovered evidence of a hot outflow streaming from Sagittarius A*.

Black holes are best known for pulling in nearby material, but researchers say they should also expel some of that matter in the form of winds or jets.

As gas spirals toward a black hole, it accelerates and heats up, generating enough energy to launch those outflows.

Astronomers have observed those outflows around many actively feeding black holes throughout the universe. However, despite more than 50 years of searching, they had never found clear evidence of a current wind coming from Sagittarius A*.

“Unless a black hole exists in a perfect vacuum, it must blow a wind somehow,” Mark Gorski, a research assistant professor at Northwestern University and study co-lead author, said in a press release. “And there is no perfect vacuum in the universe. With new observations, this is the first time we’ve had a clean enough view to see the wind’s imprint. We looked at the data and said, ‘There it is. There is the thing that everybody’s been looking for for 50 years.’”

One reason the search has been so difficult is the Milky Way itself.

“To observe our own black hole, we have to look through the plane of our galaxy,” Elena Murchikova, an assistant professor of physics and astronomy at Northwestern and co-author of the study, said in a press release. “That means we have to peer through gas, dust and ionized structures, and you can’t really see through all of that easily.”

The breakthrough came after researchers analyzed five years of observations from the Atacama Large Millimeter/Submillimeter Array, or ALMA, in Chile. After removing bright radio emissions from the black hole, they created what they described as the deepest and sharpest map yet of cold molecular gas surrounding Sagittarius A*.

The image revealed a striking feature of a cone-shaped cavity nearly three light-years long where cold gas appeared to be missing.

Researchers say the most likely explanation is a hot wind flowing from the black hole.

“If you blow hot material from the black hole, it’s not going to want to exist with the cold material,” Gorski said. “It’s either going to push the cold material out or heat it up. And, if it’s too hot, you will no longer see the cold gas.”

The team calculated that nearby stars could not have generated enough energy to carve out such a large cavity. The structure also aligned with X-ray observations from NASA’s Chandra X-ray Observatory, providing additional evidence that the black hole is responsible.

“Exceptional claims require exceptional evidence,” Gorski said. “We wanted to make sure that we weren’t just looking at some sort of imaging artifact. Then, the X-ray image from Chandra just slotted in perfectly. The molecular features lined up.”

Based on the size of the cavity and its effects on nearby gas, researchers estimate the wind has been active for at least 20,000 years.

The finding also suggests Sagittarius A* is behaving much like other supermassive black holes, even during a relatively calm period.

“The majority of other galaxies spend most of their lives in a state where they are not particularly active,” Murchikova said. “But we can only see them when they are in a fireworks stage. It is very attractive to study black holes when they are in the fireworks stage, but that’s not actually their dominant state. Sagittarius A* finally gives us a window into the life of a black hole in this quiet state.”