There is a new addition to the portrait gallery of black holes of astronomers.
On May 12, astronomers announced that they had finally collected an image of the supermassive black hole at the center of our galaxy.
“This image shows a bright ring surrounding the darkness, a telltale sign of the shadow of the black hole,” said astrophysicist Ferial Ozel of the University of Arizona at Tucson at a news conference announcing the result.
The black hole, known as Sagittarius A *, looks like a faint silhouette among the luminous material that surrounds it. The image reveals the turbulent, twisting region that surrounds the black hole with new details. The findings were also published on May 12 6 studies in Astrophysical Journal Letters.
A planetary network of radio telescopes known as the Event Horizon Telescope is working together to create this long-awaited 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). This object is located in the center of the galaxy M87, about 55 million light years from Earth.
But Sagittarius A *, or Sgr A * for short, is “the black hole of humanity,” said astrophysicist Sera Markoff of the University of Amsterdam and a member of the EHT collaboration.
27,000 light-years away, the hippopotamus is the closest giant black hole to Earth. This proximity means that Sgr A * is the most studied supermassive black hole in the universe. Yet Sgr A * and the like remain some of the most mysterious objects ever found.
This is because, like all black holes, Sgr A * is an object so dense that its gravitational attraction does not allow light to escape. Black holes are “natural keepers of their own secrets,” said physicist Lena Murchikova of the Princeton Institute for Advanced Study in New Jersey, who is not part of the EHT team. Their gravity captures light that falls within a boundary called the event horizon. Sgr A * EHT images and the black M87 hole to this inevitable edge.
Sgr A * feeds on hot material pushed by massive stars in the galactic center. This gas, attracted to Sgr A * by its gravitational attraction, flows into a surrounding disk of luminous material called an accretion disk. The disk, the stars and the outer X-ray balloon are “like an ecosystem,” said astrophysicist Daryl Haggard of McGill University in Montreal and a member of the EHT Collaboration. “They’re completely tied together.”
This accretion disk is where the action is – because the gas moves in extremely strong magnetic fields – so astronomers want to know more about how the disk works.
Like most supermassive black holes, Sgr A * is quiet and weak (SN: 6/5/19 ). The black hole eats only a few bites from its accretion disk. Still, “it’s always been a bit of a puzzle as to why it’s so, so weak,” said astrophysicist Meg Uri of Yale University, who is not part of the EHT collaboration. The M87’s black hole, by comparison, is a monster that swallows up nearby material and fires huge, powerful jets (SN: 11/10/21). But this does not mean that Sgr A * does not produce light. Astrophysicists have seen its region dimly lit by radio waves, flickering in infrared rays and belching in X-rays.
In fact, the accretion disk around Sgr A * appears to be constantly vibrating and simmering. This variability, the constant oscillation, is like foam on the ocean waves, says Markov. “So, we see this foam that comes from all this activity, and we try to understand the waves under the foam.
The big question, she adds, is whether astronomers will be able to see something change in these waves with EHT. In the new work they have seen hints of these changes under the foam, but the full analysis is still ongoing.
By combining about 3.5 petabytes of data, or the equivalent of about 100 million TikTok videos, taken in April 2017, researchers could begin assembling the picture. To extract an image from the original massive data mix, the EHT team needed years of work, sophisticated computer simulations, and observations in different types of light from other telescopes.
This “multi-wave” data from other telescopes was crucial for assembling the image. “By looking at these things at once and all together, we are able to come up with a complete picture,” says theorist Gibwa Musoke of the University of Amsterdam.
The variability of Sgr A *, constant boiling, complicates the analysis because the black hole changes in time scales in just a few minutes, changing as researchers plot it. “It was like trying to take a clear picture of a running child at night,” astronomer Jose L. Gomez of the Instituto de Astrofísica de Andalucía in Granada, Spain, told a news conference announcing the result. M87 was easier to analyze because it changed over the weeks.
Ultimately, a better understanding of what’s going on in the disk so close to Sgr A * could help scientists learn how many other similar supermassive black holes work.
New EHT observations also confirm the mass of Sgr A *, which is 4 million times greater than that of the sun. If the black hole replaced our sun, the shadow depicted by EHT would be in Mercury’s orbit.
The researchers also used the image of Sgr A * to test the general theory of relativity (SN: 2/3/21). Einstein’s constant theory of gravity has passed: the size of the shadow coincides with the predictions of the general theory of relativity. By testing the theory in extreme conditions – such as those around black holes – scientists hope to identify all the hidden weaknesses.
Scientists have previously tested the general theory of relativity by tracking the motions of stars orbiting very close to Sgr A * – a work that also helped confirm that the object is indeed a black hole (SN: 26.07.18). For this discovery, researchers Andrea Gez and Reinhard Hansel won a share in the 2020 Nobel Prize in Physics.SN: 10/6/20).
The two types of tests of general relativity complement each other, says astrophysicist Tuan Do of UCLA. “With these big physical tests, you don’t want to use just one method.” If one test seems to contradict the general theory of relativity, scientists can check for a corresponding discrepancy in the other.
However, the Event Horizon telescope tests general relativity much closer to the edge of the black hole, which may highlight the subtle effects of physics outside general relativity. “The closer you get, the better you are at looking for these effects,” said physicist Clifford Will of the University of Florida at Gainesville.
However, some researchers have criticized a similar test of general relativity performed using the EHT image of the black hole of M87 (SN: 10/1/20). This is because the test relies on relatively unstable physics assumptions about how the material revolves around a black hole, says physicist Sam Grala of the University of Arizona at Tucson. Testing the general theory of relativity in this way “would only make sense if the general theory of relativity was the weakest link,” but scientists’ confidence in the general theory of relativity is stronger than the assumptions included in the test, he said.
Sgr A *’s observations provide more evidence that the object is actually a black hole, says physicist Nicolas Younes of the University of Illinois at Urbana-Champaign. “It’s really exciting to have the first image of a black hole in our own Milky Way. It’s fantastic. “It ignites the imagination, like early pictures taken by astronauts on Earth from the moon,” he said.
This will not be the last eye-catching image of Sgr A * from EHT. Additional observations made in 2018, 2021 and 2022 are still awaiting analysis.
“This is our closest supermassive black hole,” says Haggard. “It’s like our closest friend and neighbor. And we have been studying it for years as a community. [This image is a] a really deep addition to this exciting black hole we’ve all fallen in love with in our careers. ”