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EHT Pinpoints Dark Heart of the Nearest Radio Galaxy

Astronomers have imaged the heart of Centaurus A in unprecedented detail and pinpointed the galaxy's central supermassive black hole, revealing how a gigantic jet is born. Most remarkably, only the outer edges of the jet seem to emit radiation, which challenges current theoretical models of jets.

Centaurus A obtained with Event Horizon Telescope
Radboud University; ESO/WFI; MPIfR/ESO/APEX/A. Weiss et al.; NASA/CXC/CfA/R. Kraft et al.; EHT/M. Janssen et al.

Cambridge, MA - At radio wavelengths, Centaurus A is one of the largest and brightest objects in the night sky, showing extended jet and giant radio lobes. After it was identified as one of the first known extragalactic radio sources in 1949, Centaurus A has been studied extensively across the entire electromagnetic spectrum by a variety of radio, infrared, optical, X-ray and gamma-ray observatories. At the center of Centaurus A lies a black hole with the mass of 55 million suns.

In a new paper published today in Nature Astronomy, data from the 2017 Event Horizon Telescope (EHT) observations have been analyzed to image Centaurus A in unprecedented detail.

"This allows us for the first time to see and study an extragalactic radio jet on scales smaller than the distance light travels in one day. We see up close and personal how a monstrously gigantic jet launched by a supermassive black hole is being born," says astronomer Michael Janssen of the Max Planck Institute for Radio Astronomy and Radboud University.

Compared to all previous high-resolution observations, the jet in Centaurus A has now been imaged at a tenfold higher frequency and sixteen times sharper resolution. With the resolving power of the EHT, astronomers can now link the vast radio lobes, or jets, of the source — which are 16 times the angular diameter of the Moon on the sky — to their location of origin in the center of the Centaurus A galaxy. This central region that hosts the supermassive black hole appears tiny in comparison; it is merely the size of an apple on the Moon when projected on the sky. This is a magnification factor of one billion.

"It's amazing that we can study Centaurus A now with the extreme resolution of the EHT. We have never seen this nucleus before as we didn't have suffi-ciently high resolution and we weren't looking at high enough frequencies. We can finally see the core from which the jets seen on large angular scales must have originated," says Maciek Wielgus, a study co-author and researcher at the Center for Astrophysics | Harvard & Smithsonian and the Black Hole Initiative. "Centaurus A is our close neighbor in the intergalactic sense; it's only about 10 million light years away. And its central region is very different from our own galactic center; there’s so much more power emerging from the nu-cleus of Centaurus A."

Understanding jets

Supermassive black holes that reside in the center of galaxies like Centaurus A feed off gas and dust that is attracted by their enormous gravitational pull. This process releases massive amounts of energy and causes the galaxy to become 'active.' Most matter lying close to the edge of the black hole falls in. Some of the surrounding particles, however, escape moments before capture and are blown far out into space, creating jets — one of the most mysterious and energetic features of galaxies.

Astronomers have relied on various models of how matter behaves near black holes to better understand this process. But they still do not know exactly how jets are launched from their central region and how they can extend over scales that are larger than their host galaxies without dispersing.

The EHT aims to resolve this mystery.

The new image released today shows that the jet launched by Centaurus A is brighter at the edges compared to the center. This phenomenon is not new but has never been observed so pronounced in a jet before.

"Theoretically, jets can collide with galactic gas and heat up the edge, but the details of such a process so close to the black hole are a complete mystery," says Koushik Chatterjee, a study co-author and researcher at the Center for Astrophysics and the Black Hole Initiative. “The brightness contrast between the center and the edge could potentially provide us with new insights about the plasma physics both within and around jets, making Centaurus A an exciting target for next-generation black hole simulations."

Future observations

The researchers predict that future observations at an even shorter wave-length and higher resolution will be able to photograph the central black hole of Centaurus A. This will require the use of space-based satellite observatories.

"These data are from the same observing campaign that delivered the famous image of the black hole in M87. The new results show that the EHT provides a treasure trove of data on the rich variety of black holes and there is still more to come," says Heino Falcke, EHT board member and professor for astrophysics at Radboud University.


To observe the Centaurus A galaxy with unprecedentedly sharp resolution at a wavelength of 1.3 mm, the EHT collaboration used Very Long Baseline Inter-ferometry (VLBI), the same technique with which the famous image of the black hole in M87 was made. An alliance of eight telescopes around the world joined together to create the virtual Earth-sized Event Horizon Telescope. The EHT collaboration involves more than 300 researchers from Africa, Asia, Eu-rope, North and South America.

The EHT consortium consists of 13 stakeholder institutes: the Academia Sinica Institute of Astronomy and Astrophysics, the University of Arizona, the Univer-sity of Chicago, the East Asian Observatory, Goethe-Universitaet Frankfurt, Institut de Radioastronomie Millimétrique (MPG/CNRS/IGN), Large Millimeter Telescope, Max Planck Institute for Radio Astronomy, MIT Haystack Observa-tory, National Astronomical Observatory of Japan, Perimeter Institute for The-oretical Physics, Radboud University and the Center for Astrophysics | Harvard & Smithsonian.

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