At a distance of about 12 million light-years from Earth, the supermassive black hole in the galaxy Centaurus A is ejecting two giant streams of plasma - jets - at a speed close to the speed of light. These jets are brighter at their edges than in the center. For a long time the reason remained unclear, but an explanation has been proposed by a team from the Department of Astronomy at the University of Wurzburg. The work has been published in the journal Astronomy & Astrophysics.
Centaurus A is the active galaxy closest to us, and its black hole is about 60 million times more massive than the Sun. Because of this proximity, astrophysicists can examine the jets in exceptional detail, so the galaxy serves as a unique laboratory for studying extreme conditions in the Universe.
In theory, a jet should be brightest at its center. However, observations with the Event Horizon Telescope (EHT) show the opposite: in Centaurus A it is mainly the edges that shine. This phenomenon has occupied researchers for decades.
The Wurzburg team found an explanation in the behavior of plasma - the ionized gas of which the jets consist. In the extremely strong magnetic fields of the jet, fast electrons move along the field lines. This changes the character of their radiation: brightness depends on the orientation of the magnetic field. Because the magnetic field in the jet has a complex structure, bright edges and a comparatively dark core arise - exactly the picture recorded by the EHT.
"Cen A is our nearest window into the physics of relativistic jets," says Felix Glaser, a master's student in the Department of Astronomy and the first author of the study. "The fact that our model, which takes plasma into account, reproduces the observed edge brightening means that we have grasped an important piece of the underlying physics. These processes may not be unique to Cen A, but may occur in relativistic jets throughout the Universe."
It is especially interesting that the simulation links processes on a tiny particle scale with structures extending over several light-days. It not only reproduces the bright edge structure but also agrees exceptionally well with EHT images made at a wavelength of 1.3 millimeters.
"It is striking that microscopic plasma effects are capable of shaping structures on enormous scales," notes Dr. Christian Fromm, head of the computational astrophysics group. "Future observations will reveal even more such plasma signatures, and that is why it is already becoming increasingly important to include realistic plasma physics in our models."
The study helps to understand how black holes accelerate particles to colossal energies and what radiation arises in the process. It is also an example of the work of the DFG research group "Relativistic Jets in Active Galaxies" (FOR 5195), which brings together observations, modeling and plasma physics.
The study of black hole jets is entering a new era. New millimeter telescopes, including the planned Wetterstein Millimeter Telescope on the Zugspitze, will make observations noticeably sharper. Major projects such as the next-generation Very Large Array and the Square Kilometre Array also promise even more detailed data in the future.
"A black hole and a thermonuclear reactor may sound like completely different worlds, but plasma physics rules in both," says Professor Karl Mannheim, head of the Department of Astronomy at the University of Wurzburg. "In Centaurus A we can study plasma under conditions that cannot be created on Earth."