Active Galactic Nuclei (AGN) can form powerful collimated relativistic outflows, called jets, which
carry a fraction of the total gravitational energy released during the accretion of matter onto a
supermassive black hole (SMBH) at the center of the AGN. First noticed by Heber D. Curtis
in 1918, AGN jets emit brightly across the electromagnetic spectrum, often exhibit apparent
superluminal motion, and are involved in the regulation of star formation and galaxy evolution
via AGN feedback in the interstellar medium (ISM). Even beyond the galaxy scale, AGN jets can
play a dominant role in heating the intracluster medium (ICM) and cause a substantial impact
on the evolution of galaxy clusters and cosmological structure formation.
The formation of jets addresses long-standing fundamental physics questions related to the
extraction of rotational energy from the central SMBH and the formation of hydromagnetic flows
from accretion disks around such compact objects. Imaging of AGN jets is now possible in the
radio band using Very Long Baseline Interferometry (VLBI) at millimeter wavelengths, yielding
unprecedented angular resolution of the jet-launching zone. It is now clear that AGN jets are
emerging from SMBHs with foot points as small as a few gravitational radii, and very-high-energy
gamma-ray observations have demonstrated the existence of emission regions even smaller
than one gravitational radius.
Very recently, AGN jets have been associated with very-high energy neutrinos and are among
the most likely candidates for the long sought-after sources of ultrahigh-energy cosmic rays,
which have also recently been shown to be of extragalactic origin.
These impressive observational breakthroughs within only a few years have moved AGN
jets into the focus of modern astroparticle physics. However, fundamental questions regarding
their composition, formation, collimation, and dissipation are still open: How are relativistic jets
launched from the vicinity of supermassive black holes? How are they accelerated and collima-
ted? Which radiative and dynamic processes govern the high-energy emission of AGN jets and
what are AGN jets composed of? How do AGN jets interact with their host galaxy? How does
AGN feedback heat the ICM and which are the observational signatures?
In this talk, I will present how we are addressing these questions within a coordinated network
across various research groups in Germany. This includes theory, modeling, observation, and
interpretation of all relevant angles of AGN-jet physics to deliver answers to these questions in
the light of game-changing new observational information collected by the latest astronomical
facilities at all observational wavelengths.