From ALMA(CL): “EHT pinpoints dark heart of the nearest radio galaxy”


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An international team anchored by the Event Horizon Telescope (EHT) Collaboration, which is known for capturing the first image of a black hole in the galaxy Messier 87, has now imaged the heart of the nearby radio galaxy Centaurus A in unprecedented detail. The astronomers pinpoint the location of the central supermassive black hole and reveal how a gigantic jet is being born. Most remarkably, only the outer edges of the jet seem to emit radiation, which challenges our theoretical models of jets. This work, led by Michael Janssen from the MPG Institute for Radio Astronomy [MPG Institut für Radioastronomie](DE) in Bonn and Radboud University Nijmegen [Radboud Universiteit](NL) is published in Nature Astronomy on July 19th.

At radio wavelengths Centaurus A emerges as one of the largest and brightest objects in the night sky. 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, which is right between the mass scales of the Messier 87 black hole (six and a half billion suns) and the one in the center of our own galaxy (about four million suns).

In a new paper in Nature Astronomy, data from the 2017 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 personally how a monstrously gigantic jet launched by a supermassive black hole is being born”, says astronomer Michael Janssen.

Highest resolution image of Centaurus A obtained with the Event Horizon Telescope on top of a color composite image of the entire galaxy. Credit: Radboud University; European Southern Observatory [Observatoire européen austral][Europäische Südsternwarte] (EU) (CL)/WFI; MPG Institute for Radio Astronomy [MPG Institut für Radioastronomie](DE) A. Weiss et al./ESO/APEX/; National Aeronautics Space Agency (US)/Chandra X-ray Center (US)/ R. Kraft et al.Harvard Smithsonian Center for Astrophysics (US)/; M. Janssen et al. EHT.


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European Southern Observatory/National Radio Astronomy Observatory(US)/National Astronomical Observatory of Japan(JP) ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres.[/caption]

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California Institute of Technology Owens Valley Radio Observatory(US), located near Big Pine, California (US) in Owens Valley, Altitude1,222 m (4,009 ft).

Caltech Owens Valley Radio Observatory.

Compared to all previous high-resolution observations, the jet launched in Centaurus A is imaged at a tenfold higher frequency and sixteen times sharper resolution. With the resolving power of the EHT, we can now link the vast scales of the source, which are as big as 16 times the angular diameter of the Moon on the sky, to their origin near the black hole in a region of merely the width of an apple on the Moon when projected on the sky. That is a magnification factor of one billion.

Understanding jets

Supermassive black holes residing in the center of galaxies like Centaurus A are feeding off gas and dust that is attracted by their enormous gravitational pull. This process releases massive amounts of energy and the galaxy is said to become ‘active’. Most matter lying close to the edge of the black hole falls in. However, some of the surrounding particles escape moments before capture and are blown far out into space: Jets – one of the most mysterious and energetic features of galaxies – are born.

Astronomers have relied on different models of how matter behaves near the black hole to better understand this process. But they still do not know exactly how jets are launched from its central region and how they can extend over scales that are larger than their host galaxies without dispersing out. The EHT aims to resolve this mystery.

The new image shows that the jet launched by Centaurus A is brighter at the edges compared to the center. This phenomenon is known from other jets, but has never been seen so pronouncedly before. “Now we are able to rule out theoretical jet models that are unable to reproduce this edge-brightening. It’s a striking feature that will help us better understand jets produced by black holes”, says Matthias Kadler, TANAMI leader and professor for astrophysics at the Julius Maximilian University of Würzburg [Julius-Maximilians-Universität Würzburg] (DE) in Germany.

Future observations

With the new EHT observations of the Centaurus A jet, the likely location of the black hole has been identified at the launching point of the jet. Based on this location, the researchers predict that future observations at an even shorter wavelength and higher resolution would 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.

Additional Information

To observe the Centaurus A galaxy with this unprecedentedly sharp resolution at a wavelength of 1.3 mm, the EHT collaboration used Very Long Baseline Interferometry (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, of which ALMA is the most sensitive element, joined together to create the virtual Earth-sized Event Horizon Telescope. The EHT collaboration involves more than 300 researchers from Africa, Asia, Europe, North and South America.

The EHT consortium consists of 13 stakeholder institutes: the Academia Sinica Institute of Astronomy and Astrophysics, the University of Arizona (US), the University of Chicago (US), the East Asian Observatory, Goethe University [Goethe-Universität] Frankfurt(DE), Institut de Radioastronomie Millimétrique (MPG/CNRS/IGN), Large Millimeter Telescope, MPG Institute for Radio Astronomy [MPG Institut für Radioastronomie](DE), MIT Haystack Observatory (US), National Astronomical Observatory of Japan [国立天文台](JP), Perimeter Institute for Theoretical Physics (CA), Radboud University Nijmegen [Radboud Universiteit](NL) and the Center for Astrophysics | Harvard Smithsonian Center for Astrophysics (US).

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The Atacama Large Millimeter/submillimeter Array (ALMA) (CL) , an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile. ALMA is funded in Europe by the European Organization for Astronomical Research in the Southern Hemisphere (ESO) (EU), in North America by the U.S. National Science Foundation (NSF) in cooperation with the National Research Council of Canada (NRC) (CA) and the National Science Council of Taiwan (NSC) and in East Asia by the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Academia Sinica (AS) in Taiwan.

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