Radio galaxies belong to a class of galaxies with an active and luminous core in the centre from which huge, directed matter flows (jets) are ejected. Extremely massive black holes with a mass of hundred million to several billion solar masses are suggested as the energy source of these jets. The jets from active galactic nuclei can be explained by the infall of matter onto such supermassive black holes. They throw particles with speeds close to the velocity of light over hundreds of thousands of light years into the universe. Because of the large distance to these objects, jet structures can only be resolved for the closest ones.
In the case of the radio galaxy Centaurus A, which is (on astronomical scales) only about 12 million light years away and the brightest galaxy in the southern sky, its jet was so far measured only from radio wavelengths to X-ray energies. “At the most energetic end of the electromagnetic spectrum Centaurus A could only be observed as an unresolved point source”, explains Dr. Markus Holler from the Institute of Astro and Particle Physics of the University of Innsbruck and member of the H.E.S.S. collaboration. Whether the most energetic gamma radiation was emitted by the galactic nucleus or by the jet was not possible to unravel in this way.
This is where an important contribution from the astro(particle) physicists in Innsbruck comes in: Just like for the recently recorded extension of the Crab nebula in very high energy gamma-rays, the scientists from the working group of Univ.-Prof. Olaf Reimer used a new simulation environment for the investigation of the structure of Centaurus A. It allowed a much more precise analysis of the data set of the H.E.S.S. telescopes, in total consisting of more than 200 hours of observation time.
“In our field, simulations are crucial for the analysis, but are typically generated without direct connection to the observations. However with our new simulation environment, a custom-tailored simulation is generated for each individual observation”, as laid out by Markus Holler. This is how very high energy gamma-rays along the jet of Centaurus A were detected. “It is now not only possible to distinguish Centaurus A from a point source for the first time, but also to associate the emission with the jet by using its shape.” The measured extension of the jet of more than 2 arcminutes thereby reveals an exciting secret of Centaurus A - namely the answer to the question where and how the highest-energy gamma-rays in Centaurus A emerge.
Conclusion on particle acceleration in the jet
By combining our knowledge on the radio, X-ray and gamma-ray radiation along the extended jet to a complete picture, one can consistently explain the emission measured with H.E.S.S. only with an extremely efficient particle acceleration mechanism operating along the jet. “The sole fact that photons from the jet could be recorded in this very-high-energy gamma radiation regime requires the existence of charged particles that must reach energies of at least 10 to 100 trillion electronvolt”, highlights theoretician Prof. Anita Reimer from the Institute for Astro and Particle Physics of the University of Innsbruck. “To which extent similarly efficient particle acceleration in extended jet regions is a characteristic feature also of other active galactic nuclei which do not stick out by their extreme radio brightness like Centaurus A is a question that the future Cherenkov Telescope Array (CTA) should be able to answer”, supposes Prof. Olaf Reimer, who coordinates the Austrian contribution to the preparations and building of the highest-energy gamma-ray observatory CTA. Future research will thus tell if Centaurus A can be seen as generic for many more galaxies.
Indirect measurement of gamma-ray radiation
The H.E.S.S. telescopes are named after the discoverer of cosmic rays and Nobel laureate Victor Franz Hess, who worked as Professor at the University of Innsbruck from 1931 until 1937. They measure gamma radiation which is about 1.000 billion times more energetic than optical light. Each of these gamma quanta produces (amongst other) a multitude of charged particles when hitting the Earth’s atmosphere which in turn emit optical light via the Cherenkov effect (a kind of optical analog to the supersonic bang). The H.E.S.S. telescopes are operated by an international collaboration in Namibia since 2002. Since 2009 Austria is also a member, Olaf Reimer from the Institute for Astro and Particle Physics of the University of Innsbruck leads the Austrian H.E.S.S. group.
Publication: The H.E.S.S. Collaboration: “Resolving acceleration to very high energies along the Jet of Centaurus A” in Nature, 17. Juni 2020. DOI: 10.1038/s41586-020-2354-1