Active Galactic Nuclei
Galactic nuclei are the domicile of the enigmatic super-massive black holes. Observations constrain their masses to millions
to billions of solar masses, measured by their gravitational pull. These very interesting objects cause a number of
interesting phenomena, which our group is interested in. Black holes are believed to be seeded at much smaller masses
in galaxies in the early universe. They grow on Cosmological timescales, in recurrent active phases, where strong gas
accretion takes place. The gas spirals into the hole in an accretion disk which releases the lost potential energy of
the gas as broadband light emission. Active galactic nuclei easily outshine their host galaxies. We study the Cosmological
evolution of Active Galactic Nuclei statistically by semi-analytic Cosmological modelling (see picture on the left hand side).
This may, for example help
to constrain the process that is responsible for the remarkable in-activity of the most massive galactic nuclei in the
recent Cosmological past.
Our black hole feeding simulations together with current observations by the Very Large
Telescope have found substantial evidence that nearby black holes are fed from the stellar ejecta of their also recently
formed nuclear star clusters. The picture on the right hand side shows the formation of a parsec scale disk, predicted by these simulations,
which is indeed found in several systems by observations with the Very Large Telescope Interferometer.
The active black holes, triggered
in this way, emit intense radiation. Some cloudy and dusty systems (Broad Line Region and Torus) of dense gas remain close
to the black hole in spite of the very strong outward radiation pressure. We investigate by grid and particle based
simulations, how the radiation pressure acts on the parsec-scale environment.
One issue is that the effect might
influence measurements of the black hole mass, another is that it might help to make the torus geometrically thick.
The lower left movie shows the orbit of a Broad Line Region cloud which is heavily supported by anisotropic radiation pressure.
The anisotropy causes exchange of orbital energy with the radiation field, which makes the orbits periodically shrink
and expand. The second observable besides the mass is the spin of a black hole. It could be responsible for the
production of bipolar jets. Such jets may interact with the host galaxy e.g. via heating of its gaseous halo. The
movies on the lower right hand side show different morphologies of the resulting radio sources due to the
interaction with their environment (FR II - upper movie, FR I lower movie).
M.G.H.Krause, M.Hirschmann, M.Schartmann