Into the Dark Heart of Centaurus A

Event Horizon Telescope exposes the center of the nearest radio galaxy

July 19, 2021

An international team anchored by the Event Horizon Telescope (EHT) Collaboration known for capturing the first image of a black hole in the galaxy M87 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. This is challenging theoretical models of jets. The work, led by Michael Janssen from the Max Planck Institute for Radio Astronomy in Bonn and Radboud University Nijmegen, is published in Nature Astronomy on July 19, 2021.

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 (with the galaxy NGC 5128), 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 M87 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.

Centaurus A was probed at 1-mm wavelength by record-breaking observations with a single pair of telescopes in January 2015, as it was observed by the APEX telescope and the radio telescope at the South Pole.  “These pioneering observations, from which we could only estimate the compactness of the heart of the source, paved the way to the image we present now, once the full EHT array has been deployed”, adds astronomer Eduardo Ros from the Max-Planck-Institut für Radioastronomie (MPIfR).

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 connect the vast dimensions of Centaurus A. The large-scale radio source shows an extent as big as 16 times the angular diameter of the Moon on the sky. The origin of the jet near the black hole occurs in a region of merely the width of an apple on the Moon when projected on the sky. That corresponds to a magnification factor of one billion (1 000 000 000 or 109).

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, leader of the TANAMI program and professor for astrophysics at the University of Würzburg 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”, says Heino Falcke, EHT board member and professor for Astrophysics at Radboud University.

Anton Zensus, director at MPIfR and founding board chairman of the EHT collaboration, is confident: “The EHT does not only reveal black hole shadows, it also probes their powerful jets in detail.  Relativity and magnetic fields are at work in a black hole´s jet, which we can observe. We are now investigating these magnetic fields in the heart of radio galaxies and quasars. I am sure we will soon master the advanced methods needed to analyze new observations.”


Additional Information:

To observe the Centaurus A galaxy with this unprecedentedly sharp resolution at a wavelength of 1.3 mm, the Event Horizon Telescope (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 (ALMA and APEX (Chile), IRAM-30m (Spain), JCMT and SMA (Hawaii), LMT (Mexico), SMT (Arizona) and the SPT at the South Pole) joined together to create the virtual Earth-sized EHT, later-on extended by NOEMA (France), GLT (Greenland) and KPT (Arizona). 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, the University of Chicago, the East Asian Observatory, Goethe-Universität Frankfurt, Institut de Radioastronomie Millimétrique (MPG/CNRS/IGN), Large Millimeter Telescope, Max Planck Institute for Radio Astronomy (MPIfR), MIT Haystack Observatory, National Astronomical Observatory of Japan, Perimeter Institute for Theoretical Physics, Radboud University and the Center for Astrophysics | Harvard & Smithsonian.

The EHT is supported by considerable international investment and a big commitment from the MPI für Radioastronomie since the 1990s in the development of the millimetre-VLBI technique.  Additional funding came through the ERC-funded Black Hole Camera project, for which Michael Kramer, director at the MPIfR, is one of the three principal investigators.  A new ERC-project, quoted “Mapping Magnetic Fields with INterferometry Down to Event hoRizon Scales” (M2FINDERS), led by Anton Zensus at the MPIfR (grant number 101018682) will probe over the next years the role of magnetic fields near black holes, focusing on several radio galaxies and quasars at millimetre wavelengths with the EHT and the Global mm-VLBI Array, among them Centaurus A.  

The Atacama Pathfinder Experiment (APEX), which was essential in the pioneering 2015 observations and in the work presented here, is a collaboration between MPIfR, Onsala Space Observatory (OSO), and the European Southern Observatory (ESO) to construct and operate a modified prototype antenna of ALMA (Atacama Large Millimetre Array) as a single dish on the Chajnantor plateau at an altitude of 5,100 metres above sea level (Atacama Desert, Chile). The telescope was manufactured by VERTEX Antennentechnik in Duisburg, Germany. The operation of the telescope is entrusted to ESO.

TANAMI (Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry) is a multiwavelength program to monitor relativistic jets in active galactic nuclei of the Southern Sky.  This program has been monitoring Centaurus A with VLBI at centimeter-wavelengths since the mid 2000s. The TANAMI array consists of nine radio telescopes located on four continents observing at wavelengths of 4 cm and 1.3 cm.

The following thirty-four researchers affiliated to MPIfR are co-authors of the paper (listed in order of appearance): Michael Janßen, Eduardo Ros, Thomas Krichbaum, Jun Liu, Cornelia Müller, Walter Alef, Rebecca Azulay, Anne-Kathrin Baczko, Silke Britzen, Ralph P. Eatough, Ramesh Karuppusamy, Dong-Jin Kim, Jae-Young Kim, Michael Kramer, Rocco Lico, Kuo Liu, Andrei P. Lobanov, Ru-Sen Lu, Nichola Marchili, Karl M. Menten, Nicholas R. MacDonald, Aristeidis Noutsos, Gisela N. Ortiz-León, Felix M. Pötzl, Helge Rottmann, Alan L. Roy, Tuomas Savolainen, Lijing Shao, Pablo Torné, Efthalia Traianou, Jan Wagner, Norbert Wex, Robert Wharton, and J. Anton Zensus.

Detailed description of Fig. 1: Distance scales uncovered in the Centaurus A jet. The top left image shows how the jet disperses into gas clouds that emit radio waves, captured by the Australia Telescope Compact Array (ATCA) and Parkes observatories. The top right panel displays a color composite image, with a 40x zoom compared to the first panel to match the size of the galaxy itself. Submillimeter emission from the jet and dust in the galaxy measured by the LABOCA/APEX instrument is shown in orange. X-ray emission from the jet measured by the Chandra spacecraft is shown in blue. Visible white light from the stars in the galaxy has been captured by the MPG/ESO 2.2-metre telescope. The next panel below shows a 165 000x zoom image of the inner radio jet obtained with the TANAMI telescopes.
The bottom panel depicts the new highest resolution image of the jet launching region obtained with the EHT at millimeter wavelengths with a 60 000 000x zoom in telescope resolution. Indicated scale bars are shown in light years and light days. One light year is equal to the distance that light travels within one year: about nine trillion kilometers. In comparison, the distance to the nearest-known star from our Sun is approximately four light years. One light day is equal to the distance that light travels within one day: about six times the distance between the Sun and Neptune.

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