At the heart of every large galaxy is a supermassive black hole millions or billions of times the mass of the Sun. And whenever astronomers study these black holes in neighboring galaxies, they spot intense winds of hot gas billowing out from their event horizons. Yet despite decades of scrutiny, the black hole at the heart of our own Galaxy seemed strangely quiet.
Now, researchers have finally found evidence of winds emanating from the Milky Way’s black hole: a curious cone-shaped region in which cold gases appear to have been blown away, they report in a preprint paper posted to arXiv last month. The discovery could close one of the most puzzling mysteries of the past half-century of astronomy.
“If this is true, then it would be a very exciting discovery with some pretty broad implications for the center of our Galaxy,” says Lia Hankla, a postdoctoral astrophysicist at the University of Maryland. However, she notes, it would be helpful to first have “a piece of evidence for [the wind] and not just an absence of something,” such as an outflow of particles whose velocity could be measured.
When gas spirals into the disk of material that surrounds a supermassive black hole, it heats up enormously. Rather than falling into the black hole, some of this gas instead gets blasted out through a combination of thermal, radiation, and magnetic effects. If the magnetic effects are strong, the wind can also take the form of laserlike jets of plasma that rocket out of the poles of the black hole at nearly the speed of light. These hot winds stir up surrounding cold gas, sweeping it away from the black hole and sometimes even out of the galaxy. The effect quenches star formation by making it harder for large clumps of gas to collapse into stellar seeds.
But finding the winds from our Galaxy’s black hole, called Sagittarius A* (Sgr A*), has become a white whale—largely because of the difficulty of peering through a veil of stars, gas, and dust at the galactic center. Although the Event Horizon Telescope was able to zoom in and image Sgr A* in 2022, the telescope’s planet-size lens has too sharp a focus to reveal much about the black hole’s surroundings.
Northwestern University astrophysicist Lena Murchikova and astronomer Mark Gorski thought they might have a chance at spotting the winds using Chile’s Atacama Large Millimeter/submillimeter Array (ALMA), which can penetrate clouds of dust. The 66-antenna radio array can detect not only hot, ionized gas, but also the faint glow of cold molecular gas, which emits light at specific frequencies.With about 100 hours of ALMA observations, the duo obtained some data on carbon monoxide (CO), a gas not looked for in the galactic center since the 1990s because its emissions have been assumed to be too messy to show anything useful. “Maybe we’ll look at it if we have time,” Gorski recalls thinking. When they got around to mapping the CO gas near the black hole, the data yielded a surprise: a distinct, cone-shaped gap.
The team overlaid x-ray data from the Chandra X-ray Observatory on its CO map, finding that it neatly traced the same cone shape. That led the researchers to conclude that a previously undetected hot plasma wind was pushing cold gas away from its path and emitting x-rays as it blew. The wind points out of the plane of the galactic disk at a 45° angle and spans a length of about 3 light-years. That angle also implies that Sgr A* is spinning out of alignment with the orientation of the galactic plane, the researchers say.
The finding affirms that “our place in the universe is not unique,” Murchikova says. Now, researchers need to try to “figure out how these [winds] are generated” from matter spinning close to the black hole in more detail, she adds.
Read the full story by Hannah Richter in Science.
Feature image: A cone-shaped cavity defined by an absence of cold gas points to a hot plasma wind blowing out from Sagittarius A*. M. D. Gorski/E. Murchikova