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The last image of a black hole captures the huge jets of Centaurus A.

The Horizon Telescope Event is expanding its portfolio of black hole images.

In 2019, the telescope presented the first image of a black hole, revealing the supermassive beast 55 light-years from Earth in the center of the galaxy M87 (SN: 4/10/19). That faded orange ring showed the shadow of the black hole in its shiny accretion disk of falling material. Since then, observations from the Event Horizon Telescope, or EHT, have given more detailed views of the M87 black hole (SN: 23/09/20). Now, EHT data has revealed new details of the supermassive black hole in the heart of a galaxy close to ours, called Centaurus A.

Instead of getting close enough to see the shadow of the black hole, the new image offers the clearest view of the powerful jets of plasma coming out of the black hole. This perspective gives an insight into how supermassive black holes cause these plasma jets to explode in space, researchers reported online on July 19 in Nature Astronomy.

“It’s a pretty impressive feat,” says Craig Walker, the radio astronomer, of capturing the new high-resolution image. “These (jets) are some of the most powerful things in the universe,” says Walker, of the National Radio Astronomy Observatory in Socorro, N.M., who did not participate in the work. Because such superfast plasma flows influence the way galaxies grow and evolve, astronomers are interested in understanding how jets form (SN: 29/03/19).

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The researchers pointed to the global network of radio dishes that make up the EHT in Centaurus A for six hours in April 2017, during the same observational test that delivered the first image of a black hole (SN: 4/10/19). About 12 million light-years from Earth, Centaurus A is one of the brightest galaxies in the sky and is known for the huge jets ejected from its central black hole.

“They extend virtually the entire scale of the galaxy,” says Michael Janssen, a radio astronomer at the Max Planck Institute for Radio Astronomy in Bonn, Germany. "If we saw the radio light (with our eyes) and looked at the night sky, we would see these jets of Centaurus A as a structure 16 times larger than the full moon."

Using the EHT, Janssen and his colleagues fixed themselves at the base of those jets, which protrude from either side of the black hole accretion disk. The new image is 16 times sharper than previous observations of the jets, probing details with less than a day's light, about four times the distance from the sun to Pluto. One of the most striking features the image reveals is that only the outer edges of the jets appear to glow.

jet images of the black hole of the galaxy Centaurus AThe supermassive black hole of the galaxy Centaurus A launches two plasma jets in opposite directions (reduced view of the jets on the left). In a new close-up view taken by the Event Horizon Telescope (right; estimated location of the black hole indicated by an arrow), the jet moving toward Earth points to the upper left of the image, with two bright edges and a dark center . The jet that moves away from the Earth, also bright only at the edges, points to the lower right.M. Janssen et al / Nature Astronomy 2021

“That’s still an enigma,” Janssen says. One possibility is that the jets are spinning, which can cause the material in some regions of the jets to emit light toward the Earth, while others do not. Or the jets could be hollow, Janssen says.

Recent observations from other galaxies have hinted that jets of supermassive black holes are brighter around the edges, says Denise Gabuzda, an astrophysicist at University College Cork in Ireland who was not involved in the work. "But it was hard to know if it was a common feature or if it was something quirky of the few that was observed."

The new view of the black hole of Centaurus A provides evidence that this edge glow is common, says Gabuzda. "It's pretty weird to be able to detect jets coming out in both directions, but in the Centaurus A images … you can clearly see that both are brighter at the edges."

The next step will be to compare Centaurus A’s EHT image with computer simulations based on Einstein’s general theory of relativity, to test how well relativity holds up in this extreme environment, Janssen says. Examination of the polarization, or orientation, of light waves emanating from the jets of Centaurus A could also reveal the structure of its magnetic fields – just as polarization revealed magnetism around the black hole of M87 (SN: 3 / 24/21).

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