The Milky Way's supermassive black hole has a hot spot

The orbiting spot could be a bubble of hot gas whipping around Sagittarius A* at around 30% of the speed of light.

By Robert Lea

Astronomers have spotted a bright 'hot spot' swirling around the supermassive black hole at the heart of our Milky Way galaxy, Sagittarius A* (Sgr A*).

The team behind the discovery thinks that the 'hot spot' could be a bubble of hot gas orbiting Sgr A* as fast as 30% of the speed of light. The discovery could help astronomers and astrophysicists better understand the violent environment at the center of the Milky Way, and around Sgr A* in particular.

"We think we're looking at a hot bubble of gas zipping around Sgr A* on an orbit similar in size to that of the planet Mercury, but making a full loop in just around 70 minutes," Maciek Wielgus, an astrophysicist at the Max Planck Institute for Radio Astronomy in Germany, said in a statement(opens in new tab). "This requires a mind-blowing velocity of about 30% of the speed of light!"

Sgr A.jpeg
An image of the supermassive black hole at the center of the Milky Way, a behemoth dubbed Sagittarius A*, revealed by the Event Horizon Telescope on May 12, 2022. (Image credit: Event Horizon Telescope collaboration)

sgr A 2.jpeg
Sagittarius A*, as seen by the Event Horizon Collaboration (EHT) with an illustration of the hot spot seen by astronomers. (Image credit: EHT Collaboration, ESO/M. Kornmesser (Acknowledgment: M. Wielgus))

Wielgus led a team that collected observational data using the Atacama Large Millimeter/submillimeter Array (ALMA) telescope, comprised of 66 radio antennas spread across the Atacama Desert of northern Chile, as part of the Event Horizon Telescope (EHT) collaboration's work to image black holes.

Along with other telescopes in the EHT, ALMA started observing supermassive black holes in 2017. This led to the first-ever image of a black hole, released in 2019, which depicted the supermassive black hole at the heart of the galaxy Messier 87 (M87). Earlier this year, the same collaboration unveiled the first image of Sgr A*.

But ALMA recorded additional data at the same time as the EHT observations of Sgr A*. Wielgus and his team found within that data clues to the nature of Sgr A* and its surroundings, buried in the measurements made by only ALMA.

The discovery comes because ALMA collected some of its data after a burst, or flare, of X-rays from the heart of the Milky Way detected by NASA's Chandra X-ray Observatory. Scientists have previously linked flares like this to magnetic interactions in hot gas bubbles that orbit close to Sgr A* at rapid speeds.

"What is really new and interesting is that such flares were so far only clearly present in X-ray and infrared observations of Sagittarius A*," Wielgus said. "Here we see for the first time a very strong indication that orbiting hot spots are also present in radio observations."

The team suggests that the hot spots detected at infrared wavelengths could be the result of gas bubbles that become visible at longer wavelengths of light (like those ALMA sees) when they cool down.

"Now we find strong evidence for a magnetic origin of these flares and our observations give us a clue about the geometry of the process," Monika Mościbrodzka, an EHT collaboration team member and an astrophysicist at Radboud University in the Netherlands, said in the same statement. "The new data are extremely helpful for building a theoretical interpretation of these events."

Using ALMA, astronomers and astrophysicists can study polarized radio wave emissions from Sgr A*, which they can use to investigate the magnetic field surrounding the supermassive black hole. The new research could help in this investigation by better constraining the shape of this magnetic field and details of the surroundings of Sgr A*, the scientists hope.

Additionally, the results help confirm previous research based on data from the GRAVITY instrument on the Very Large Telescope (VLT) in Chile, which implied that X-ray flares come from clumps of gas swirling clockwise around black holes at 30% the speed of light.

The team now hopes that both GRAVITY and ALMA can track these hot spots in multiple wavelengths of light, which could be a milestone in the understanding of the physics of flares at the center of the Milky Way and which would build on direct observations of Sgr A* and its environment by the EHT.

"Hopefully, one day, we will be comfortable saying that we 'know' what is going on in Sgr A*," Wielgus concluded.

A paper detailing the team's findings is published in the September issue of the journal Astronomy & Astrophysics.



One of the foremost observers of Sagittarius A* is Andrea Ghez of UCLA and her team.

Andrea M. Ghez, professor of Physics & Astronomy and Lauren B. Leichtman & Arthur E. Levine chair in Astrophysics at UCLA, is one of the world’s leading experts in observational astrophysics and heads UCLA’s Galactic Center Group.

Best known for her ground-breaking work on the center of our Galaxy, which has led to the best evidence to date for the existence of supermassive black holes, she has received numerous honors and awards including the Nobel Prize in 2020, she became the fourth woman to be awarded the Nobel Prize in Physics, sharing one half of the prize with Reinhard Genzel (the other half of the prize being awarded to Roger Penrose). The Nobel Prize was awarded to Ghez and Genzel for their Independent discovery of a supermassive compact object, now generally recognized to be a black hole, in the Milky Way's galactic center, the Crafoord Prize in Astronomy from the Royal Swedish Academy of Science (she is the first woman to receive a Crafoord prize in any field), Bakerian Medal from the Royal Society of London, a MacArthur Fellowship, election to the National Academy of Sciences, the American Academy of Arts & Sciences, and the American Philosophical Society.

Her work on the orbits of stars at the center of the Milky Way has opened a new approach to studying black holes and her group is currently focused on using this approach to understand the physics of gravity near a black hole and the role that black holes plays in the formation and evolution of galaxies.

Advances in high resolution imaging technology enabled Ghez’s work and her group continues to work on pushing the frontiers of these technologies forward. She serves on several leadership committees for the W. M. Keck Observatory, which hosts the largest telescopes in the world, and the future Thirty Meter Telescope.

Ghez is also very committed to the communication of science to the general public and inspiring young girls to get into science. Her work can be found in many public outlets including TED, NOVA’s Monster of the Milky Way, Discovery’s Swallowed by a Black Hole, TED, and Griffith Observatory.

Ghez earned her B.S from MIT in 1987, and her PhD from Caltech in 1992 and has been on the faculty at UCLA since 1994.

The following is from Hilton Lewis, Director of W.M. Keck Observatory on October 8, 2020:

Standing in my office 25 years ago was an unknown, newly minted astronomer with a half-smile on her face. She had come with an outrageous request—really a demand—that my team modify our exhaustively tested software to make one of our most important and in-demand scientific instruments do something it had never been designed for and risk breaking it.

All to carry out an experiment that was basically a waste of time and couldn’t be done—to prove that a massive black hole lurked at the center of our Milky Way.

My initial "no way" (perhaps I used a stronger expression) gradually gave way in the face of her cheerful but unwavering determination. It was my first encounter with a force of nature, Andrea Ghez, one of three winners of the 2020 Nobel Prize in Physics, for her work on providing the conclusive experimental evidence of a supermassive black hole with the mass of four million suns residing at the center of the Milky Way galaxy.

That determination and the willingness to take calculated risks have always characterized Andrea. For 25 years she has focused almost exclusively on Sagittarius A*—the name of our own local supermassive black hole. It is remarkable that an entire field of study has grown up in the intervening quarter century—of searching for and finding evidence of these monsters thought to lie at the heart of every large galaxy. And Andrea is without question one of the great pioneers in this search.

andrea ghez ucla.jpg
Andrea Ghez


It is hard for me to comprehend, oftentimes, that we can optically observe our own supermassive black hole, located in the center of the Milky Way. Many of us have heroes in science, and Andrea Ghez is one of mine for her doggedly persistent and groundbreaking work on Sagittarius A*, our galaxy's own central black hole and her use of the stars orbiting Sagittarius A* to determine the black hole's mass. Someday, I'd love to attend an Andrea Ghez lecture.
  • Like
Reactions: JAXX1 and write4u