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The massive jets of matter springing from the center of Centaur A are fueled by a power-hungry black hole.
NASA / CXC / SAO; Rolf Olsen; NASA / JPL-Caltech; NRAO / AUI / NSF / University of Hertfordshire / M. Hardcastle
By Daniel Cléry
The astronomy team that 2 years ago captured the first close-up of a giant black hole lurking in the center of the Messier 87 (M87) galaxy, has now zoomed in on a second, somewhat smaller giant in the nearby active galaxy Centaurus A The latest image from the Event Horizon Telescope (EHT) is expected to help answer questions about how such galactic centers channel huge amounts of matter into powerful beams and project them thousands of years ago -light in space. Together, the images also support theorists’ belief that all black holes work the same, despite huge variations in their masses.
“It’s really good,” astronomer Philip Best of the University of Edinburgh said of the new EHT image. “The angular resolution is amazing compared to previous images of these jets.”
The EHT merges dozens of widely dispersed satellite dishes, from Hawaii to France and Greenland to the South Pole, into a huge virtual telescope. By pointing a large number of dishes at a celestial object at the same time and carefully time-stamping each other’s data with an atomic clock, researchers can then reassemble it with massive computational clusters – a process that takes years – to produce a image with a resolution as sharp as that of a simple satellite dish the size of the Earth. One of the challenges is to simultaneously obtain observation time on 11 different observatories, so that the EHT only operates a few weeks each year; bad weather and technical problems often reduce this window even further.
The virtual telescope probed Centaurus A during the same 2017 observation campaign that produced the now famous image of the M87 supermassive black hole—ScienceBreakthrough of the Year for 2019. Centaurus A, about 13 million light years away, is one of the closest galaxies to Earth that glows at radio wavelengths. It also has obvious jets spewing material above and below the galactic disk, a feature of an active giant black hole. “We wanted to see what the jet looked like at the resolution” that the EHT could offer, says Michael Janssen, team member at the Max Planck Institute for Radio Astronomy. “We didn’t know what to expect.
The Event Horizon telescope produced detailed images of the material beams from the center of Centaurus A, revealing that the jets have a dark center parallel to bright edges.
Mr. Janssen, Nature astronomy (2021) 10.1038
The result, which he and his colleagues report today in Nature astronomy, was a detailed image of how the jet emerges from the region around the Centaurus A supermassive black hole, showing remarkable similarity to EHT images of the M87 jet at a much smaller scale. Images of Centaurus A jets taken by other telescopes at different wavelengths revealed little detail, but EHT images show the jet with a dark center flanked by two light bands; Best suggests that the jet may appear shiny around its edge because its outer regions rub against the surrounding gas and dust, causing them to glow.
Astrophysicists don’t fully understand how galactic nuclei drive these incredibly powerful fountains. One theory holds that an accretion disk, the vortex of matter swirling in the black hole, generates a magnetic field that channels some of the matter in a jet. Others argue that this magnetic field must tap into the rotational energy of the black hole itself in order to achieve such colossal power.
The new sightings of Centaur A do not resolve this question, but they do contain clues. Janssen says the images show the remarkably parallel edges of the jet tapering into a cone close to the black hole. The base of this cone remains wide, he says, which could suggest that it originated from the accretion disc. “That remains to be seen,” he said.
Theoretical astrophysicist Jim Beall of St. John’s College says there may not be a single answer: “It’s a symbiotic relationship,” he says. “The EHT takes us close to the accretion disc. The results are really beautiful.
Centaurus A images also fill a big gap in black hole observations. Observers have studied the operation of jets from the largest black holes, including M87, weighing billions of times the mass of our Sun. They also saw jets coming from much smaller black holes, with masses of a few dozen Suns. The new view of Centaurus A, at 55 million times the mass of the Sun, looks like its larger and smaller parents. This confirms the idea that black holes are essentially simple objects, defined only by their mass, charge, and spin, so those with the mass of a large star should not behave any differently than those with the mass of a galaxy.
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