She are something like cosmic vacuum cleaners and particle guns in one: super-massive black holes in the center of galaxies do not only attract matter. Some of the gas masses drawn are thrown back into space in gigantic “jets”. The reason for this lies in the dynamics of the attracted matter, which heats up enormously as it rolls in. In this hot plasma, a complex interplay of magnetic fields ensures that two closely collimated, counter-rotating particle beams are formed, which can reach thousands of light years into space. So far, however, little is known about exactly how such jets form and spread, and an important missing piece of the puzzle of modern astrophysics – because such jets are responsible for a substantial part of the cosmic radiation.
A team of researchers from the HESS consortium (“High Energy Stereoscopic System”) has now made a surprising discovery that seems to upset some previous assumptions. In “Nature”, the scientists describe that they used their telescope system to monitor the Centaurus A radio galaxy for a total of 200 hours in the area of gamma radiation – this is extremely high-energy electromagnetic radiation that is many orders of magnitude more energetic than X-rays. Centaurus A shows two extensive radio lobes, which are driven by the two jets of the central black hole. The galaxy obtains its high luminosity in the radio and also in the gamma range from the fact that its black hole is particularly active and swallows up matter to a high degree. Centaurus A is the closest radio galaxy at a distance of around twelve million light years and is therefore particularly suitable for detailed observational studies.
Treacherous traces of light from cosmic gamma quanta
Some radio galaxies are known to be strong sources of gamma. But so far it has never been possible to resolve the spatial structure of the jets in the gamma range. Due to the large cosmic distances, they could only be recognized as point sources with the corresponding telescopes. This is because gamma radiation cannot be imaged optically as easily as radiation in the visible, infrared or radio range. The HESS team, consisting of 200 researchers from 13 countries, had to push the special technology of their telescope system – currently the most powerful of its kind – to the limits of its capabilities.
Your observatory consists of several telescopic shells, which do not look into space, but look at the upper atmosphere. If one of the rare, extremely high-energy gamma quanta of cosmic radiation hits the uppermost layers of the air, a light trail is created for a brief moment, which these telescopes can record from different viewing angles and thereby determine the direction of origin of the gamma quantum.
So far, astrophysicists have assumed that high-energy gamma radiation mainly comes from the central region of the galaxy directly near the black hole. This seemed to follow from the fact that the strength of the gamma emissions can vary every minute. This is only possible if the emission region is spatially limited.
Shock waves from galactic jets
“As we have now been able to determine for the first time, the high-energy gamma quanta are not only generated in a small area, but over thousands of light years in a jet,” says Mathieu de Naurois from the École Polytechnique in Palaiseau. In order to achieve the sharpness achieved in the gamma range and to extract as much information as possible from the data, the researchers had to develop very complex analysis programs.
The discovery that the gamma emission extends across the jet is very revealing for the research of cosmic radiation, because the gamma quanta are secondary products in such jets. The strong magnetic fields initially accelerate electrons to extreme energies – far higher than the energies that are possible with earth’s particle accelerators. The electrons then release part of their energy in the form of gamma radiation. However, since high-energy electrons lose their energy at a distance of around 100 light years due to scattering effects with the interstellar matter and the background radiation, this means that highly effective acceleration mechanisms must also work far out in the jet.
“So the electrons are not simply accelerated in the immediate vicinity of the black hole and then carry this energy outwards with the jet,” explains de Naurois, “You also experience enormous accelerations along the jet.” How that could work physically had the Nobel laureate Enrico Fermi explained about 70 years ago with so-called relativistic shock fronts in a turbulent plasma. However, it was not yet clear to what extent such shock fronts can form along galactic jets. The new measurements should therefore make it much easier to model these particle guns.
However, there is still the question of whether Centaurus A may have rather unusual jets in this regard. The researchers therefore now want to examine the radio galaxy M87 – one of the few other radio galaxies in cosmic proximity. The gamma structure of more distant radio galaxies will only become visible with the even more powerful “Cherenkov Telescope Array”, which is currently under construction.