EHT pinpoints dark heart of the nearest radio galaxy

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At the centre of Centaurus A is a black hole with the mass of 55 million suns. T

At the centre of Centaurus A is a black hole with the mass of 55 million suns. This is the place where a massive jet is born. (Image: R. Bors; 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.; TANAMI/C. Müller et al.; EHT/M. Janßen et al.)


Centaurus A, one of the closest active galaxies to Earth, belongs to the brightest objects in the sky. An international team has now imaged the heart of Centaurus A in unprecedented detail. Scientists from the University of Würzburg were involved.

An international team around 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, which challenges our theoretical models of jets. This work, led by Michael Janssen from the Max Planck Institute for Radio Astronomy in Bonn and Radboud University Nijmegen, is now published in Nature Astronomy.

A black hole as massive as 55 million suns

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 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.

Previous studies at the University of Würzburg

This breakthrough is based on previous studies at the chair for astronomy at the Julius-Maximilians-Universität Würzburg (JMU). Already back in 2011, a spectacular image of the jets in Centaurus A was published by Cornelia Müller, who was a doctoral researcher at the time and later postdoc in the working group of Professor Matthias Kadler, professor for astrophysics at the JMU and leader of the TANAMI project. This abbreviation stands for "Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry".

Compared to previous high-resolution observations, the jet launched in Centaurus A is now imaged at a tenfold higher frequency and sixteen times sharper resolution. With the resolving power of the EHT, scientists can now link the gigantic scales of the source, which are as big as 16 times the diameter of the moon on the sky, to their origin in a region of merely the size 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 don’t 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 pronounced 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.

"In the next years, we will study jets in Würzburg via novel numerical simulations of jet launching and emission calculations", says Dr. Christian Fromm (Harvard University) who will start as a junior research group leader at JMU in October.

New EHT data of Centaurus A and other jets observed by the EHT will be studied also as part of the new DFG research unit "Relativistic Jets in Active Galaxies" that has very recently been installed for the next four years. The group’s spokesperson is Matthias Kadler at JMU in Würzburg.

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.

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