Zoom into the dark heart of Centaurus A

The Event Horizon Telescope takes a close-up of the nearest radio galaxy

Centaurus is one of the most famous constellations in the southern sky. Within this constellation is the radio galaxy Centaurus A, which can be seen as a faint nebula using only binoculars. Like most galaxies, Centaurus A is also home to a supermassive black hole. With the Event Horizon Telescope (EHT), researchers have now zoomed into the heart of this galaxy some 13 million light years away. In the process, the team not only precisely determined the position of the black hole but also observed a gigantic jet originating there. This bundled gas stream appears to emit radiation only at its outer edges, thereby calling theoretical models into question.

Centaurus A shines in the radio range as a massive and bright object on the earthly firmament. In 1949, the galaxy with the catalogue number NGC 5128 was identified as one of the first extragalactic radio sources. Since then, astronomers have explored them extensively across the electromagnetic spectrum – not only in optical light and radio waves but also with infra-red, X-ray, and gamma-ray telescopes.

Centaurus A hides a black hole with a mass 55 million times that of the sun. In comparison, the gravity trap in the centre of the galaxy has a good four million solar masses, and the black hole in the giant elliptical galaxy M 87 has about six and a half billion. The latter was examined by the EHT in April 2017; the image of the shadow of this black hole, published two years later, has made a huge contribution to scientific history.

In 2017, the eight antennas of the EHT, which are distributed around the globe, also collected data from Centaurus A. After the complex analysis, the core region of the galaxy now appears in previously unattained detail. “This allows us for the first time to study an extragalactic radio jet on scales smaller than the distance light travels in a day”, says team leader Michael Janssen of the Max Planck Institute for Radio Astronomy in Bonn and Radboud University Nijmegen. “We see first-hand, how a massive jet is born from a supermassive black hole”.

Centaurus A had already been thoroughly scanned once: at a wavelength of one millimetre by both the Atacama Pathfinder Experiment (APEX) and the South Pole Telescope (STP) in January 2015. “These ground-breaking measurements – from which we were able to estimate the compactness of the nucleus – paved the way for the picture we have now obtained by using the complete EHT network”, says Eduardo Ros of the Max Planck Institute for Radio Astronomy.

In terms of sharpness, the measurement with the EHT at a wavelength of 1.3 millimetres surpasses all previous observations many times over. The origin of the jet near the black hole at an angle corresponds to the size of an apple on the moon – a magnification factor of one billion. This is made possible by a special method called Very Long Baseline Interferometry (VLBI). The signals that the different observatories receive from one and the same object at the same time are superimposed. The resolving power of such a “virtual antenna” is similar to that of an earth-sized radio telescope.

Super-massive black holes located at the centres of active galaxies like Centaurus A exert an almost irresistible pull on their surroundings. They feed on gas and dust and release huge amounts of energy during their “meal”. Most of the matter near the edge of a black hole falls into the cosmic void. However, some of the surrounding particles escape just before capture. This creates “jets”, the mechanism of which is still a mystery.

Researchers are using different models to try to explain exactly how matter behaves near a black hole. But how are the jets launched from the galactic centres? And how can they extend many thousands of light years out into space, far surpassing their host galaxies in size? The EHT aims to help answer these questions.

For example, the new image shows that the jet originating in the interior of Centaurus A is brighter at the edges than in the centre. Scientists know this phenomenon from other jets. But it had never been observed quite so clearly. “With this striking feature, we can now rule out all theoretical jet models from which no such edge brightening results”, says Matthias Kadler, an astrophysicist at the University of Würzburg.

In addition, the EHT measurements have identified the likely position of the black hole at the starting point of the jet. Based on this finding, the researchers predict that future observations at even shorter wavelengths and higher detail resolution will make it possible to depict the black hole at the heart of Centaurus A – analogous to that in the giant galaxy M 87.

“The EHT allows us to not only look at the shadows of black holes but also investigate the origin of the giant jets of matter in galaxies”, says Anton Zensus, Director at the Max Planck Institute for Radio Astronomy and founding chair of the EHT collaboration. “In the jets emerging from the immediate vicinity of the black hole, relativity and magnetic fields interact”.

According to Zensus, research with the EHT will now focus more on the magnetic fields at the heart of radio galaxies and quasars – young, active star systems. “I am sure that we will soon master the improved methods needed to evaluate the new observations”.


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