Acronym for Active Galactic Nuclei,
almost exclusivly used in the scientific literature
instead of the longer version. An AGN is thought
to turn on when a lot of gas falls onto
a
black hole in the centre of a
galaxy. The gas forms an
accretion disk and heats up and radiates
according to
thermal bremsstrahlung radiation.
Half of the potential energy of the material falling into the black hole has to be converted into kinetic energy, this is due to the virial theorem. This kinetic energy
is the heating source of the disk.
Material dropping onto the black hole can get whipped up
into huge radio jets that can be many times the size of the host galaxy. An exact mechanism for this
has not been settled on, an example of a solution
from fluid mechanics is the Blanford-Rees nozzle jet model.