A Steady Breeze from the Milky Way’s Black Hole
Astronomers may have solved a long-standing puzzle surrounding the giant black hole lurking at the center of our Galaxy, Sagittarius A* (Sgr A*). Using observations from the Atacama Large Millimeter/submillimeter Array (ALMA) radio observatory in Chile, Mark Gorski and Lena Murchikova of Northwestern University in Illinois have obtained signatures of a previously elusive wind from Sgr A* [1]. Such a wind had long been predicted but was never convincingly observed. If confirmed, the discovery could offer a rare glimpse into how the majority of supermassive black holes interact with their surroundings.
Supermassive black holes grow by accreting dust, stars, and gas in their vicinity. Nothing can escape the event horizon, but the turbulent environment outside of the horizon can propel gas outward in the form of jets and winds. Giant jets of plasma—such as the one emerging from M87*, the first black hole ever imaged—can be launched by twisted magnetic fields acting like cosmic slingshots. The same environment can drive less collimated outflows, or winds, through a combination of magnetic forces, thermal pressure, and radiation. Loosely speaking, “jets are to winds what laser pointers are to flashlights,” Gorski says.
Much of our understanding of these phenomena comes from the most extreme objects, such as quasars and active galactic nuclei, where winds and jets have been extensively documented. But most supermassive black holes, including Sgr A*, are thought to display less fireworks, living in quiescent, low-luminosity states that are far more difficult to study. Theory still predicts that these “quiet” black holes should fuel winds, but decades of observations of the closest black hole in the sky could not confirm such predictions. “The absence of a wind was one of the most uncomfortable things about our own Galaxy’s black hole,” Murchikova says.
To hunt for this missing wind, the duo mapped gas around the Galactic Center using ALMA radio observations at 230 GHz, a frequency that selectively traces cold carbon monoxide gas. Their analysis produced the least noisy and highest-resolution map ever—a 100-fold sensitivity gain over previous efforts. According to the researchers, the key to this imaging quality was the painstaking subtraction of background radiation from Sgr A*, which is hundreds of thousands of times brighter than the gas and fluctuates rapidly. This subtraction involved estimating the black hole’s emission every 15–30 seconds.
The 230-GHz map revealed a giant cone-shaped cavity in the cold molecular gas pointing directly away from Sgr A*. Additional clues to interpreting the observation came from a comparison with data from NASA’s Chandra X-ray Observatory. While the cavity is clearly visible in ALMA’s cold-gas map, it does not appear in Chandra’s x-ray images. Instead, the same region is filled with hot, x-ray-emitting plasma. According to the researchers, this pattern indicates that a wind from the black hole has either swept away the cold molecular gas or heated it until it no longer emits 230-GHz radiation. The ALMA observations thus revealed where the cold gas was missing, while Chandra showed what had taken its place.
The researchers say that alternative interpretations can be ruled out. In particular, the cavity is unlikely to be a chance fluctuation in the molecular gas distribution, given its correspondence with independent signatures at other wavelengths. Nor do winds from nearby massive stars provide a convincing explanation, as they would be too weak and too broadly distributed to carve the sharply defined cone.
Gorski and Murchikova say the significance of the discovery extends beyond the Milky Way. Until now, astronomers have mostly learned about black hole outflows by studying what Murchikova calls the cosmic equivalent of hurricanes—rare and spectacular events that are easy to detect. “But there aren’t thunderstorms or hurricanes every day,” Gorski says. “Wind, on the other hand, blows every day.” The gentle breeze emanating from Sgr A* may be representative of the more common ways supermassive black holes shape their surroundings.
–Matteo Rini
Matteo Rini is the Editor of Physics Magazine.
References
- M. D. Gorski and L. Murchikova, “The discovery of an active wind from the Milky Way’s central black hole,” Astrophys. J., Lett. 1004, L7 (2026).