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July 19, 2021
from the Dutch Research School for Astronomy
An international team from the Event Horizon Telescope (EHT) Collaboration, known for capturing the first image of a black hole in the galaxy Messier 87, has now mapped the heart of the next radio galaxy, Centaurus A, in unprecedented detail. The astronomers locate the location of the central supermassive black hole and show how a gigantic jet is created. Remarkably, only the outer edges of the jets appear to be emitting radiation, which calls into question our theoretical jet models. This work, led by Michael Janssen from the Max Planck Institute for Radio Astronomy in Bonn and Radboud University Nijmegen, will be published in Nature Astronomy on July 19.
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At radio wavelengths, Centaurus A appears as one of the largest and brightest objects in the night sky. After being identified as one of the first known extragalactic radio sources in 1949, Centaurus A was extensively studied across the electromagnetic spectrum by a variety of radio, infrared, optical, X-ray, and gamma-ray observatories. In the center of Centaurus A lies a black hole with the mass of 55 million suns, which lies exactly between the masses of the black hole Messier 87 (six and a half billion suns) and Sgr A * in the center of our own galaxy (about four million suns).
In a new article in Nature Astronomy, data from 2017 EHT observations were analyzed to depict Centaurus A in unprecedented levels of detail. “This enables us for the first time to see and study an extragalactic radio jet on smaller scales than light travels in a day. We see up close and personally how a monstrous, gigantic jet is created, which is launched by a supermassive black hole. « Says the astronomer Michael Janssen.
Compared to all previous high-resolution observations, the jet launched in Centaurus A is also included a ten times higher frequency and a 16 times sharper resolution. With the resolving power of the EHT, astronomers now associate the huge scales of the source, which are as large as 16 times the angular diameter of the moon in the sky, with their origin near the black hole in an area only the width of an apple on the moon, when projected onto the sky. That is a magnification factor of one billion.
Super massive black holes in the center of galaxies like Centaurus A feed on gas and dust, which are drawn by their enormous gravitational pull. This process releases enormous amounts of energy and the galaxy is supposed to become « active ». Most of the matter that is close to the edge of the black hole falls into it. However, some of the surrounding particles escape shortly before being captured and are blown far into space: Jets – one of the most mysterious and energetic properties of galaxies – are born.
Astronomers have relied on various models of how nearby matter dwell of the black hole to better understand this process. But they still don’t know exactly how jets are launched from their central region and how they can extend across scales larger than their host galaxies without dissipating. The EHT wants to solve this riddle.
The new picture shows that the jet launched by Centaurus A is brighter at the edges than in the middle. This phenomenon is known from other jets, but has never been observed in such a pronounced manner. “We found it difficult to explain this in terms of the same models that we used for the M87. Something else has to happen, like spiral magnetic fields that give us new clues on how to ‘squeeze’ the jets, ”says Sera Markoff, Vice Chair, EHT Science Council and Professor of High Energy Theoretical Astrophysics at the University of Amsterdam .
With the new EHT observations of the Centaurus A jet, the probable position of the black hole at the starting point of the jets was identified. Based on this location, the researchers predict that future observations at even shorter wavelengths and higher resolution could photograph Centaurus A’s central black hole. This requires the use of space-based satellite observatories.
« These data come from the same observation campaign that produced the famous image of the black hole in M87. The new results show that the EHT offers a treasure trove of data on the rich diversity of black holes and more is to come. » says Heino Falcke, EHT board member and professor of astrophysics at Radboud University.
In order to observe the galaxy Centaurus A with this unprecedented sharp resolution at a wavelength of 1.3 mm, the EHT collaboration used the very long baselines -Interferometry (VLBI), the same technique that was used to take the famous image of the black hole in M87. An alliance of eight telescopes around the world have come together to create the virtual Earth-sized Event Horizon Telescope. More than 300 researchers from Africa, Asia, Europe, North and South America are involved in the EHT collaboration.
TANAMI (Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry) is a multi-wavelength program for monitoring relativistic jets in active galactic cores 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 on four continents, which observe at wavelengths of 4 cm and 1.3 cm.
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