Astronomers’ Portrait of Black Holes has a new addition to the gallery.
Astronomers announced on May 12 that they have finally assembled an image of the supermassive black hole at the center of our galaxy.
“This image shows a bright ring around the dark, the telltale sign of a black hole’s shadow,” astrophysicist Feriel Ozell of the University of Arizona in Tucson said at a news conference announcing the results.
The black hole, known as Sagittarius A*, appears as a faint silhouette amidst the glowing material around it. The image immediately reveals in new detail the turbulent, twisting region surrounding the black hole. The findings were also published on May 12 in 6 Studies astrophysical journal letters,
A planet-spanning network of radio telescopes, known as the Event Horizon Telescope, worked together to create this much-anticipated look at the Milky Way giant. Three years ago, the same team released the first image of a supermassive black hole (SN: 4/10/19) that object is located at the center of the galaxy M87, about 55 million light-years from Earth.
But Sagittarius A*, or SGR A* for short, is “humanity’s black hole,” says astrophysicist Sera Markoff of the University of Amsterdam and member of the EHT collaboration.
At 27,000 light-years away, the behemoth is the closest supermassive black hole to Earth. That proximity means that Sgr A* is the most studied supermassive black hole in the universe. Yet Sgr A* and others like it are some of the most mysterious objects ever found.
This is because, like all black holes, Sgr A* is such a dense object that its gravitational pull will not allow light to escape. Black holes are “natural keepers of their own secrets,” says physicist Lena Murchikova of the Institute for Advanced Study in Princeton, NJ, who is not part of the EHT team. Their gravity traps light that falls within a boundary called the event horizon. EHT’s images of the Sgr A* and M87 black hole skirt that inevitable edge.
Sgr A* feeds on hot material pushed by the giant stars at the galactic center. The gas, pulled toward Sgr A* by its gravitational pull, flows into a surrounding disc of glowing material called the accretion disc. The disk, the star and an outer bubble of X-ray light “is like an ecosystem,” says astrophysicist Daryl Haggard of McGill University in Montreal and a member of the EHT collaboration. “They’re totally tied together.”
That accretion disk is where the action happens—as gas moves in extremely strong magnetic fields—so astronomers want to learn more about how the disk works.
Like most supermassive black holes, Sgr A* is quiet and faint (SN: 6/5/19 ) The black hole eats only the few morsels fed to it by its accretion disk. Still, “it has always been a bit of a puzzle as to why it is so faint,” says astrophysicist Meg Urie of Yale University, who is not part of the EHT collaboration. M87’s black hole, in comparison, is a monster that chugs around and ejects huge, powerful jets of material (SN: 11/10/21) but that doesn’t mean that Sgr A* isn’t producing light. Astrophysicists have observed its region glowing weakly in radio waves, jittery in infrared, and dazzling in X-rays.
In fact, the accretion disk surrounding Sgr A* appears to be constantly twinkling and shrinking. This variability, the constant twinkling, is like a foam on the ocean waves, Markoff says. “And so we’re looking at the foam coming out of all this activity, and we’re trying to understand the ripples beneath the foam.”
She says the big question is whether astronomers will be able to see something changing in those waves with the EHT. In the new work, he has seen signs of those changes under the foam, but the full analysis is still ongoing.
By combining the roughly 3.5 petabytes of data captured in April 2017, or the equivalent of about 100 million TikTok videos, researchers can begin to piece together the picture. To tease out an image from the initial massive perturbation of the data, the EHT team required years of work, complex computer simulations and observations in a variety of light from other telescopes.
“Multiwavelength” data from other telescopes were critical to assembling the image. “By looking at these things at once and all at once, we are able to come up with a complete picture,” says theorist Gibwa Musoke from the University of Amsterdam.
The variability, continual simmering, of Sgr A* complicates analysis because the black hole changes on timescales of just a few minutes, changing as the researchers were imaging it. “It was like trying to get a clear picture of a kid running at night,” said astronomer Jose L., of the Instituto de Astrofísica de Andalucía in Granada, Spain. Gomez said at a news conference announcing the results. M87 was easy to analyze as it changed over the course of weeks.
Ultimately, a better understanding of what’s happening in the disk so close to Sgr A* could help scientists learn more about how many other similar supermassive black holes are at work.
The new EHT observations also confirm the mass of SGR A* at 4 million times that of the Sun. If the black hole replaced our Sun, the shadow EHT image would sit within the orbit of Mercury.
The researchers also used an image of Sgr A* to put general relativity to the test (SN: 2/3/21Einstein’s firm theory of gravitation came to pass: the shape of the shadow matched the predictions of general relativity. By testing the theory in extreme conditions – such as around black holes – the scientists hope to pinpoint any hidden weaknesses.
Scientists have previously tested general relativity by following the motion of stars that orbit very close to Sgr A* – a work that also helped confirm that the object is indeed a black hole (SN: 7/26/18) For that discovery, researchers Andrea Gage and Reinhard Genzel won a share of the 2020 Nobel Prize in Physics (SN: 10/6/20,
The two types of tests of general relativity are complementary, says UCLA astrophysicist Tuan Do. “With these big physics tests, you don’t want to use just one method.” If one test appears to contradict general relativity, scientists can investigate the same discrepancy in another.
The Event Horizon Telescope, however, tests general relativity very close to the edge of the black hole, which may uncover subtle implications of physics beyond general relativity. “The closer you get, the better you are in terms of being able to see these effects,” says physicist Clifford Will of the University of Florida in Gainesville.
However, some researchers have criticized the similar test of general relativity performed using an EHT image of M87’s black hole (SN: 10/1/20) That’s because the test relies on relatively unstated assumptions about the physics of how material moves around a black hole, says physicist Sam Graalla of the University of Arizona in Tucson. Testing general relativity in this way “makes sense only if general relativity were the weakest link,” but scientists’ belief in general relativity is stronger than the assumptions that went to the test, he says.
The observations of SGR A* provide more evidence that the object is actually a black hole, says University of Illinois Urbana-Champaign physicist Nicholas Younes. “It’s really exciting to have the first image of a black hole in our own galaxy. It’s great.” It sparks the imagination, as astronauts took early pictures of Earth from the Moon, he says.
This won’t be the last flashy image of Sgr A* from EHT. Additional observations made in 2018, 2021 and 2022 are still awaiting analysis.
“It’s our closest supermassive black hole,” Haggard says. “It is like our closest friend and neighbour. And we as a community have been studying it for years. [This image is a] A really deep association with this exciting black hole we’ve loved every step of our career. ,