Northwestern Team Detects the Milky Way Black Hole's Long-Predicted Wind, Ending a 50-Year Search
ALMA observations reveal a cone-shaped cavity carved into cold gas around Sagittarius A*, the first clear evidence of a wind blowing from the galaxy's central black hole.
Overview
For half a century, theorists insisted that the supermassive black hole at the center of the Milky Way had to be blowing a wind. The galactic center never showed it. Now a team at Northwestern University reports the first clear evidence of that wind: a vast, cone-shaped cavity carved into the cold gas around Sagittarius A* (Sgr A*), pointing straight back at the black hole. According to Northwestern University, the astrophysicists detected the outflow after a 50-year search, using five years of deep observations from the Atacama Large Millimeter/Submillimeter Array (ALMA) in Chile.
The results were published in The Astrophysical Journal Letters in a paper titled “The Discovery of an Active Wind from the Milky Way’s Central Black Hole,” authored by Mark D. Gorski and Lena Murchikova, as recorded by Sci.News (doi: 10.3847/2041-8213/ae63cf).
What We Know
Sagittarius A* is the 4.3-million-solar-mass black hole that resides at the center of our Milky Way Galaxy, according to Sci.News. The team mapped emission from carbon monoxide molecules, a classic tracer of cold molecular gas, and found a large cone-shaped hole in that gas pointing straight at the black hole, the same outlet reports.
The cavity is nearly one parsec long and 45 degrees wide, devoid of cold molecular gas, according to the Northwestern announcement. That is within about three light-years of Sagittarius A*, as Sci.News notes. The five years of ALMA data — gathered with 66 radio antennas in northern Chile — produced the sharpest image of cold molecular gas in the region, per Space.com.
The resulting image is 100 times deeper and 80 times sharper than previous maps of the region, according to EurekAlert!. That depth is what finally made the cavity legible. “With new observations, this is the first time we’ve had a clean enough view to see the wind’s imprint,” Mark Gorski told Space.com.
The interpretation rests on long-standing theory. “Unless a black hole exists in a perfect vacuum, it must blow a wind somehow,” Gorski said, as quoted by Northwestern. Matter falling toward the black hole accelerates to near light-speed, generating pressure that expels infalling material, Space.com explains.
A second instrument supports the picture. NASA’s Chandra X-ray Observatory previously pinpointed bright X-ray emissions in the exact same region where the cone-shaped cavity appeared, according to EurekAlert!. Data from Chandra show hot gas filling the same region, confirming that this is a black hole-powered outflow, Sci.News reports.
The outflow appears to be a long-lived feature. The astrophysicists estimate the wind has been active for at least 20,000 years, according to EurekAlert!. Gorski described the structure as a striking emptiness: “It’s a huge absence of material. We calculated how much energy was needed to create this cavity,” he said, as quoted by Northwestern.
The study was co-led by Mark Gorski, research assistant professor at Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), and Elena Murchikova, assistant professor of physics and astronomy at Northwestern’s Weinberg College of Arts and Sciences and a CIERA member, according to Northwestern.
The same ALMA facility has repeatedly reshaped views of the galactic center; the array earlier produced its largest-ever mosaic of the Milky Way’s core, as previously reported.
What We Don’t Know
The wind is faint, and its geometry may not be fixed. “The wind is not powerful, and its direction probably wanders with time,” Murchikova said, according to Northwestern. The reporting does not specify a measured wind speed.
The detection captures Sagittarius A* in a comparatively dormant phase rather than during a violent accretion episode. “Sgr A* finally gives us a window into the life of a black hole in this quiet state,” Murchikova told Space.com. She added context on why that matters: “The majority of other galaxies spend most of their lives in a state where they are not particularly active,” she said, as quoted by Northwestern.
Why It Matters
Because Sagittarius A* is the nearest supermassive black hole, it serves as a benchmark for the quiet, low-luminosity state that most galactic-center black holes occupy for the bulk of their lives. Detecting its wind in that state offers a rare, close-up template for the feedback processes that shape galaxies far beyond the Milky Way.