The astronomy team that 2 years ago captured the first close-up of a giant black hole, lurking at the center of the galaxy Messier 87 (M87), has now zoomed in on a second, somewhat smaller giant in the nearby active galaxy Centaurus A. The Event Horizon Telescope’s (EHT’s) latest image should help resolve questions about how such galactic centers funnel huge amounts of matter into powerful beams and fire them thousands of light-years into space. Together the images also support theorists’ belief that all black holes operate the same way, despite huge variations in their masses.
“This is really nice,” astronomer Philip Best of the University of Edinburgh says of the new EHT image. “The angular resolution is astonishing compared to previous images of these jets.”
The EHT merges dozens of widely dispersed radio dishes, from Hawaii to France and from 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 the data from each one with an atomic clock, researchers can later reassemble it with massive computing clusters—a process that takes years—to produce an image with a resolution as sharp as that of a single Earth-size dish. One challenge is getting observing time on 11 different observatories simultaneously, so the EHT only operates for a few weeks each year; poor weather and technical glitches often further narrow that window.
The virtual telescope probed Centaurus A during the same 2017 observing campaign that produced the now-famous image of the supermassive black hole in M87—Science’s Breakthrough of the Year for 2019. Centaurus A, about 13 million light-years away, is one of the closest galaxies to Earth that is bright at radio wavelengths. It also has obvious jets spewing matter above and below the galactic disk, a hallmark of an active giant black hole. “We wanted to see what the jet looked like at the resolution” EHT could offer, says team member Michael Janssen of the Max Planck Institute for Radio Astronomy. “We didn’t know what to expect.”
The result, which he and colleagues report today in Nature Astronomy, was a detailed image of how the jet emerges from the region around Centaurus A’s supermassive black hole, showing a remarkable similarity to EHT’s images of M87’s jet on a much smaller scale. Images of Centaurus A’s jets taken by other telescopes at different wavelengths revealed little detail, but the EHT images show the jet with a dark center flanked by two bright stripes; Best suggests the jet may appear bright around its edge because its outer regions rub against surrounding gas and dust, causing them to glow.
Astrophysicists don’t fully understand how galactic nuclei drive these fantastically powerful fountains. One theory holds that an accretion disk, the swirling whirlpool of matter spiraling into the black hole, generates a magnetic field that funnels some of the matter into a jet. Others argue this magnetic field must tap into the rotational energy of the black hole itself to be able to achieve such colossal power.
The new observations of Centaurus A don’t resolve that question, but they hold clues. Janssen says the images show that the remarkably parallel edges of the jet narrow into a cone close to the black hole. The base of that cone remains wide, he says, which might suggest it is coming from the accretion disk. “It remains to be seen,” he says.
Theoretical astrophysicist Jim Beall of St. John’s College says there may be no single answer: The spin of the black hole drags on the innermost stable orbit of the infalling matter, which in turn affects how the accretion disk shapes and powers the jet. “It’s a symbiotic relationship,” he says. “The EHT takes us down close to the accretion disk. The results are really quite beautiful.”
The pictures of Centaurus A also fill in a size gap in black hole observations. Observers have studied the workings of jets coming from the very largest of black holes—including M87—weighing billions of times the mass of our Sun. They’ve also seen jets from much smaller black holes, with masses of a few tens of Suns. The new view of Centaurus A, at 55 million times the Sun’s mass, looks just like its bigger and smaller relatives. 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 behave no differently from one with the mass of a galaxy.