Finally proven after 50 years: the central black hole of the Milky Way also generates a wind of high-energy, fast particles – thereby confirming a long-held assumption. This is indicated by a cone-shaped gap in the cool gases around the black hole Sagittarius A*. This gap is caused by the energetic outflow from the black hole, astronomers have determined. This confirms that even largely inactive black holes can generate such winds.
When black holes actively devour matter, this is usually noticeable: the gases and debris sucked in race around the event horizon at an ever faster pace, releasing high-energy radiation and streams of accelerated particles that shoot far into space. In active galactic nuclei, such jets are visible over millions of light-years. “Such jets – or, more generally speaking, winds – should actually all have black holes,” explain Mark Gorski and Lena Murchikova from Northwestern University in Illinois.
Does Sagittarius A* also produce wind?
But at Sagittarius A*, the central black hole of the Milky Way, this typical wind seemed to be missing: gigantic bubbles of gases and radiation testify to the fact that our supermassive black hole also swallowed larger amounts of matter a few million years ago and generated strong outflows in the process. Since then, the gravity giant has appeared largely inactive, apart from a slight flickering of its light ring.
Despite almost 50 years of searching, astronomers were unable to detect the theoretically postulated “Black Hole Wind” at Sagittarius A*. One of the reasons for this is our perspective on this black hole: “To observe it, we have to look through the main plane of the Milky Way. This means that we have to look through a lot of dust, gas and ionized structures, and these are often difficult for us to penetrate,” explains Murchikova.
Telltale gap in the cool gas
That has now changed: astronomers have succeeded for the first time in detecting the long-sought particle wind of Sagittarius A*. They tracked down his signature using images from the Atacama Large Millimeter/Submillimeter Array (ALMA) in Chile. Over the course of five years, this observatory’s radio telescopes have repeatedly targeted the center of our galaxy, mapping the movements of cool molecular carbon monoxide gases around the black hole.
From this data, Gorski and Murchikova have now created the sharpest image yet of the gases orbiting closely around the black hole. This revealed: At one point the dense shell of the cool gases is broken through – there is a gap in it that is around three light years long and widens conically at a 45 degree angle. “When you discover something like this that has never been seen before, your first thought is usually not: ‘We’ve made a discovery!’ but rather: ‘Oh dear, what went wrong with my analysis?’” explains Murchikova.
Wind is still active today
But closer analyzes and comparisons with X-ray images confirmed the observation: “An enormous amount of material is missing there,” reports Gorski. This material must either have been heated up so much that it is no longer visible in the radio recordings or it must have been blown outwards, as the astronomers explain. “In our opinion, the observed gap in the CO gas must have been generated by a hot active wind emanating from Sagittarius A*,” say the researchers.
More importantly, the sharply defined edges of the cone-shaped “wind gap” in the gas indicate that the wind from the black hole is still active today, as the astronomers explain. “This wind is relatively weak and likely deflected from its original direction by interactions with the surrounding gas,” they write. This also explains the asymmetry of the gas gap.
Finally confirmed after 50 years
This is the first time in around 50 years that astronomers have succeeded in detecting an active wind at the central black hole of the Milky Way. “This is the first time we’ve had a clear enough view to see the imprint of this wind,” Gorski says. “We have discovered exactly what everyone has been looking for for 50 years.” Murchikova adds: “This confirms that our black hole is not unique.”
The detection of Sagittarius A*’s wind confirms that Sagittarius A* also follows the laws of supermassive black holes. Although the Milky Way’s central black hole is relatively inactive today and swallows little matter, it still produces an outflow of energetic particles, albeit weak. “Sagittarius A* gives us a new insight into the life of a black hole at rest,” says Murchikova. This is also valuable for researching other supermassive black holes.
Source: Mark Gorski and Lena Murchikova (Northwestern University, Illinois, USA), The Astrophysical Journal Letters, 2026; doi: 10.3847/2041-8213/ae63cf