The dark core of the nearest radio galaxy identified by the Event Horizon Telescope

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Composition of Centaurus A images in the optical (ESO/WFI) and X-ray (NASA/CXC/CfA) range. Centaurus A is a massive galaxy in the process of merging with a neighboring vortex. Credits: ESO/WFI (optical), MpIfR/ESO/Apex/A. Weiss et al. (mm); NASA/CXC/CfA/R. Kraft et al (X-ray)

A well-established international team by the Event Horizon Telescope (EHT), known for taking the first image of a black hole in the Messier 87 galaxy, has imaged the core of the nearby radio galaxy Centaurus A in unprecedented detail. Astronomers locate the central supermassive black hole and reveal how a giant jet is born. Most strikingly, only the outer edges of the aircraft emit radiation, challenging our theoretical models of aircraft. This work, led by Michael Janssen of the Max Planck Institute for Radio Astronomy in Bonn and Radboud University Nijmegen, is published in Nature Astronomy today (July 19, 2021).

At radio wavelengths, Centaurus A appears as one of the largest and brightest objects in the night sky. Having been 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, X-ray and gamma-ray observatories. At the center of Centaurus A is a black hole with a mass of 55 million suns, which is located between the mass scales of the black hole Messier 87 (six and a half billion suns) and the hole in the center of our galaxy (about four million suns).

The upper left image shows how the aircraft is scattered in gas clouds that emit radio waves, captured by the ATCA and Parkes Observatory. The upper right panel displays a color composite image, with 40x zoom compared to the first panel to match the size of the galaxy itself. Millimeter emission of galactic jet and dust measured by the LABOCA/APEX instrument is shown in orange. X-ray emission from the aircraft measured by the Chandra spacecraft is shown in blue. Visible white light from stars in the galaxy was captured by the MPG/ESO 2.2-meter telescope. The next panel below shows a 165,000 times zoom image of the inner radio jet obtained with the TANAMI telescopes. The lower panel depicts the new, higher-resolution image of the jet launch area obtained with EHT at millimeter wavelengths with a magnification of 60000000x at the telescope resolution. Scale bars indicated in light years and light days. One light year is equal to the distance light travels in one year: about nine trillion kilometres. By comparison, the distance to the nearest known star from our Sun is roughly four light years. One light day equals the distance light travels in one day: about six times the distance between the Sun and Neptune. Credit: Radboud University. CSIRO/ATNF/I.Feain et al. , R.Morganti et al. , N. Junkes et al. ; ESO/WFI; MPIfR/ESO/APEX/A. Weiss et al.; NASA/CXC/CfA/R. Kraft et al.; growth / k. Muller et al.; EHT / m. Jansen et al

In new research in Nature Astronomy, data from 2017 EHT observations imaging Centaurus A is analyzed in unprecedented detail. “This allows us for the first time to see and study an extragalactic radio jet at scales smaller than the distance traveled by light in a day. We see up close and in person how a monstrously giant jet is being generated by a supermassive black hole,” says astronomer Michael Jansen.

Compared with all previous high-resolution observations, the jet launched at Centaurus A was imaged with a frequency ten times higher and with sixteen times more accuracy. Through the analytical power of the EHT, we can now relate the source’s vast scales, which are up to 16 times the angular diameter of the Moon in the sky, to its origin near the black hole in its width only. An apple on the moon when viewed in the sky. This is a billion amplification factor.

Understanding Airplanes

Supermassive black holes at the center of galaxies like Centaurus A are fed by gas and dust that are attracted by the immense gravity. This process releases huge amounts of energy and the galaxy is said to become “active”. Most of the material in it is located near the edge of the black hole. However, some of the surrounding particles escape moments before capture and blast away into space: jets are born – one of the most enigmatic and vibrant features of galaxies.

The highest resolution image of Centaurus A acquired by the Event Horizon Telescope is on top of a composite color image of the entire galaxy. Credit: Radboud University. ESO/WFI; MPIfR/ESO/APEX/A. Weiss et al.; NASA/CXC/CfA/R. Kraft et al.; EHT / m. Jansen et al

Astronomers have relied on different models of how matter behaves near a black hole to better understand this process. But they still don’t know exactly how the jets are released from their central region and how they can extend over scales larger than their host galaxies without scattering. The EHT aims to solve this puzzle.

The new image shows that the plane launched by Centaurus A is much brighter at the edges than at the center. This phenomenon is known from other aircraft, but has not been clearly seen before. “We are now able to rule out theoretical jet models that are unable to reproduce this edge brightness. It is an amazing feature that will help us better understand the jets produced by black holes,” says Matthias Kadler, Tanami Commander and Professor of Astrophysics at the University of Würzburg in Germany.

Reference: Jul 19, 2021, Nature Astronomy.DOI: 10.1038 / s41550-021-01417-w

future notes

With new EHT observations of the Centaurus A jet, the likely location of the black hole at the jet’s launch point has been determined. Based on this location, the researchers expect that future observations with a shorter wavelength and higher resolution will be able to image the central black hole of Centaurus A. This will require the use of space-satellite observatories.

This data is from the same observational campaign that provided the famous image of M87’s black hole. The new findings show that the EHT provides a trove of data about a rich variety of black holes, and there’s still more to come, says Hino Falk, EHT board member and professor of astrophysics at Radboud University.

Background Information

To observe Centaurus A at this unprecedentedly sharp resolution at a wavelength of 1.3 mm, the EHT collaboration used Very Long Fundamental Interferometry (VLBI), the same technology with which the famous image of M87’s black hole was made. An alliance of eight telescopes around the world have joined together to create the Earth-size hypothetical Event Horizon Telescope. The EHT collaboration includes more than 300 researchers from Africa, Asia, Europe, and North and South America.

The EHT Consortium is made up of 13 stakeholder institutes: Sinica Academic Institute of Astronomy and Astrophysics, University of Arizona, University of Chicago, East Asian Observatory, Goethe University Frankfurt, Institute for Radio Astronomy Millimétrique (MPG/CNRS/IGN), Large Millimeter Telescope, Max Institute Planck Radio Astronomy, MIT Haystack Observatory, Japan National Astronomical Observatory, Ocean Institute for Theoretical Physics, Radboud University and Astrophysics Center | Harvard and Smithsonian.

TANAMI (Tracking Active Galactic Nuclei Using Austral Millisecond Interferometry) is a multi-wavelength program for observing relativistic jets in AGNs in the southern sky. This program has been monitoring Centaurus A using VLBI at centimeter wavelengths since the mid-2000s. The Tanami Array consists of nine radio telescopes located on four continents that monitor wavelengths of 4 cm and 1.3 cm.

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