Astronomers using NASA’s Chandra X-ray Observatory have discovered an ‘exhaust vent’ funneling hot gas away from Sagittarius A*, the supermassive black hole at the center of our Milky Way Galaxy. This vent is about 26,000 light-years from Earth and is connected to a chimney-like structure previously discovered that is perpendicular to the Galactic plane. The Chandra data show a cylindrical tunnel that helps funnel gas toward the outer reaches of the Milky Way. This result reveals how the Milky Way’s black hole may ingest and reject material.
This image shows a region near the center of our Milky Way Galaxy in X-ray and radio light. At the bottom of the image, near the center, is a brilliant, tangled knot of material that resembles a paint splatter. This is the brightest region in the image, and it contains the supermassive black hole at the center of our Galaxy, known as Sagittarius A*. The lower third of the image resembles an angry firestorm. Streaks of red and orange are scattered in every direction, as if a legion of embers from a fire crackled and popped into the air all at once. Flame-like structures lick toward the center from our right. Much of the image is infused with wispy blue clouds showing X-rays detected by Chandra. At a few points, the wispy blue clouds seem to form into balls of teal colored light and are known as dust halos. They are caused by X-rays from bright X-ray sources reflecting off dust surrounding the sources. These dust halos resemble underwater lights glowing in a cloudy swimming pool at night. Rising up from Sagittarius A* in the center of the image is a pillar of blue light referred to as a chimney. This chimney of hot gas is surrounded by red clouds that are filled with stars, presenting themselves as tiny red flecks. Near the top of the blue pillar is a streak of light blue, outlined by an illustrated, gray box. This streak is referred to as the chimney exhaust vent. Just to our left is another illustrated box that shows the close-up image of the chimney vent as observed by Chandra. Image credit: NASA / CXC / Universe of Chicago / Mackey et al. / NRF / SARAO / MeerKAT / SAO / N. Wolk.
The chimney begins at the center of the Milky Way and stands perpendicular to the Galaxy’s spiral disk.
Astronomers had previously identified the chimney using X-ray data from NASA’s Chandra and ESA’s XMM-Newton missions.
Radio emission detected by the MeerKAT radio telescope shows the effect of magnetic fields enclosing the gas in the chimney.
The latest Chandra data reveal several X-ray ridges roughly perpendicular to the Galactic plane.
Astronomers think these are the walls of a tunnel, shaped like a cylinder, which helps funnel hot gas as it moves upwards along the chimney and away from the Galactic center.
The newly-discovered vent is located near the top of the chimney about 700 light-years from the center of the Galaxy.
“We suspected that magnetic fields are acting as the walls of the chimney and that hot gas is traveling up through them, like smoke,” said Dr. Scott Mackey, an astronomer at the University of Chicago.
“Now we’ve discovered an exhaust vent near the top of the chimney.”
The astronomers think the exhaust vent formed when hot gas rising through the chimney struck cooler gas lying in its path.
The brightness of the exhaust vent walls in X-rays is caused by shock waves — similar to sonic booms from supersonic planes — generated by this collision.
The left side of the exhaust vent is likely particularly bright because the gas flowing upwards is striking the tunnel wall at a more direct angle and with more force than other regions.
The researchers think that the hot gas is most likely coming from a sequence of events involving material falling into Sagittarius A* and then eruptions from the black hole driving the gas upwards along the chimney, and out through the exhaust vent.
However, they do not know exactly how often the black hole is being fed.
Previous studies have indicated that dramatic X-ray flares take place every few hundred years at or near the location of the central black hole, so those could play important roles in driving the hot gas upwards through the exhaust vent.
Astronomers also estimate that Sagittarius A* rips apart and swallows a star every 20,000 years or so.
Such events would lead to powerful, explosive releases of energy, much of which would be destined to rise through the chimney vents.
“We’re not sure if this energy and heat are stoked by a large amount of material being dumped onto Sagittarius A* at once, like a bunch of logs being dumped on a fire,” said Dr. Mark Morris, an astronomer at the University of California, Los Angeles.
“Or it might come from multiple small loads being fed into the black hole similar to kindling being regularly tossed in.”
The particles and energy in the vent provide clues about the origin of two mysterious and much larger structures around the center of the Milky Way: the Fermi Bubbles seen in gamma-rays by NASA’s Fermi Gamma-ray Space Telescope and the eROSITA Bubbles, detected by ESA’s eROSITA X-ray telescope.
Both of these are pairs of structures extending thousands of light-years away from the center of the Galaxy.
They provide important information about past explosive activity near the center of the Galaxy.
The Fermi and eROSITA bubbles are both aligned with the direction of the chimney and a second X-ray chimney that begins at the Galaxy’s center and points in the opposite direction.
The funneling effects of the exhaust vent near the top of the chimney may keep the hot gas focused as it travels upwards, aiding in the formation of the coherent structure of the bubbles.
“The origin of the Fermi Bubbles and the eROSITA bubbles are some of the biggest mysteries faced by studies of the high energy radiation from our Galaxy,” said Dr. Gabriele Ponti, an astronomer at the Italian National Institute of Astrophysics.
“We’ve discovered a small structure that might play a large role in the creation of these gigantic bubbles.”
A paper on the findings was published in the Astrophysical Journal.
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Scott C. Mackey et al. 2024. X-Rays from a Central ‘Exhaust Vent’ of the Galactic Center Chimney. ApJL 966, L32; doi: 10.3847/2041-8213/ad3248
