Every large galaxy harbors a supermassive black hole at its center, each one emitting powerful winds of hot gas from its event horizon. Our galaxy should be no exception. Yet for the last 50 or so years, astronomers have been searching for winds coming from the black hole at the Milky Way’s center, and in all that time, they found nothing. Not even a gentle breeze.
Until now. In a preliminary study, a team of scientists detail the strongest evidence found yet of winds flowing from the Milky Way’s black hole, Sagittarius A*. The breakthrough findings, posted to the preprint server arXiv in September, describe a large, cone-shaped region around the black hole where cold gas appears to have been blown away.
“If this is true, then it would be a very exciting discovery with some pretty broad implications for the center of our galaxy,” Lia Hankla, a postdoctoral astrophysicist at the University of Maryland who was not involved in the study, told Science. While she notes that the missing gas is indirect evidence of the black hole’s wind, the findings are a major step forward in solving this case.
Hunting for the winds of Sagittarius A*
Contrary to popular belief, black holes don’t just suck up everything that comes too close. As gas spirals into the disk of material surrounding a supermassive black hole, it heats up. Through a complex combination of magnetic, radiation, and thermal effects, some of this gas gets belched out in the form of winds or high-speed jets of plasma.
A supermassive black hole’s winds are so powerful, they shape how its host galaxy evolves. Astronomers know, for example, that the winds help keep intergalactic gas hot and suppress star formation, limiting the galaxy from growing too big. Understanding how these dynamics are playing out at the center of the Milky Way is key to knowing how it evolved over time, and to tracing our own origin story.
Many an astronomer has searched for Sagittarius A*’s winds, but previous telescope observations have yielded conflicting results, largely because its just hard to peer through all the gas, dust, and stars that shroud the galactic nucleus.
In this new study, however, a new telescope in Chile has risen to the occasion. The Atacama Large Millimeter/Submillimeter Array (ALMA) is the most powerful radio telescope in the world. Compared to optical telescopes, it’s exceptionally good at penetrating clouds of dust.
How they did it
Astrophysicist Lena Murchikova and astronomer Mark Gorski, both of Northwestern University, combined about five years of ALMA observations with state-of-the-art data processing techniques to produce an unprecedentedly detailed map of the cold molecular gas around Sagittarius A*.
This map revealed a cone-shaped gap in the cold gas cloud. When the researchers overlaid their map onto X-ray data gathered by NASA’s Chandra X-ray Observatory, it matched the cone shape perfectly. The alignment suggests that hot plasma wind emanating from Sagittarius A* is blowing cold gas away, and emitting X-rays in the process.
The findings bring scientists closer than ever to solving the mystery of Sagittarius A*’s missing wind, but the case isn’t quite closed. Direct evidence, such as measuring the velocity of an outflow of particles from the black hole, is still proving elusive. But with the answer so tantalizingly close, astronomers are still pushing to understand the mysterious heart of our galaxy.
