Luminosity functionsGalactic 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.  AGN starburst connectionOur 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.

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If you know the answer, please contact the CAST group! 

 

 

 

 

 

 

 

 

warped diskStar formation occurs due to the collapse of dense parts of molecular clouds. To study star formation in detail it is important to also study the interstellar medium (ISM) and molecular clouds as precursors to the star formation. Once a star has formed it has a strong impact on the evolution of the surrounding medium. The UV radiation ionises the surrounding gas and creates a hot HII region. The heated gas starts to expand into the surrounding cold molecular cloud, creating a shock-wave. This shock-wave compresses the cold gas leading to pillar like structures. In the tip of those structures the formation of new stars is possible. Another important task to understand star formation in more detail is to study this process under extreme conditions. One example is the region close to the milky-way galactic centre black hole. Here tidal forces, which would disrupt a typical molecular cloud due to the deep potential, prevent star formation in the standard way. Still there are observations of a disk of stars on a sub-parsec scale near the black hole. The large scale impact of star formation on the ISM is due to supernovas. This process can heat and stir the ISM, leading to its multiphasic structure. It also enriches the ISM with elements of higher order. The so called local bubble, a region of low density gas inside which our solar system currently resides, is a interesting source to study the effect of supernova feedback. The bubble is believed to have formed by supernovas blowing out a large, peanut shaped region in the ISM. At the edges of this region, clusters of stars can form. The supernova feedback of those stars is directed towards the low density bubble (the so called champagne-effect) and enriches the bubble with heavy elements.

spiral structure

 


Stars are known to form in massive clouds of molecular hydrogen (GMCs), which are highly structured and turbulent. Understanding the origin and evolution of these clouds is a key problem in astrophysics. Here at the LMU and MPE we perform simulations of galaxies and colliding flows to better understand the nature of GMCs and their formation. Simulations of the interstellar medium (ISM) in galaxies show the formation of GMCs by the agglomeration of smaller clouds, as gas passes through the spiral arms (left). These calculations highlight how the properties of the clouds are related to their formation: the spacing of the clouds along the arm is proportional to the strength of the spiral potential, whilst the cloud rotations and internal velocity dispersions are regulated by cloud-cloud interactions and collisions. Without stellar feedback, these clouds last several 10's, or even 100 or more Myrs. We are currently performing calculations which include stellar feedback. In this case, the lifetimes of the GMCs are typically much shorter, the lifetimes and also the morphologies of the clouds governed by the star formation efficiency adopted in the calculations. 

Cosmology

It is now well accepted that the observed structure of our universe is best reproduced in the presence of cold dark matter and dark energy. Within this framework of a LCDM cosmology, structures form in a hierarchical bottom up fashion. The size, range and completeness of observational data increased recently using the latest generation of astronomical instruments, and opened the so-called period of precision cosmology. Hence, the basic parameters describing such a LCDM universe can be in principle determined with a precision of ten per cent or better. In this hierarchical picture of structure formation, small objects collapse first and then merge in a complex manner to form larger and larger structures. To a first approximation, one can study the formation of cosmic structures using N-body simulations which follow the gravitational collapse of collision-less particles. However, with the possible exception of gravitational lensing, observations mainly reflect the state of the ordinary (baryonic) matter. Therefore, their interpretation in the framework of cosmic evolution requires that we understand the complex, non-gravitational, physical processes which determine the evolution of the cosmic baryons. The evolution of each of the underlying building blocks within the hierarchical formation scenario will contribute to the state and composition of the intra-cluster medium by being seed for metal pollution, magnetic fields, high energy particles and energy. Depending on their origin, these components will be blown out by jets, winds or ram pressure effects and finally mixed with the surrounding intra-cluster medium. Some of these effects will be naturally followed within hydrodynamic simulations (like ram pressure effects), others have to be improved via effective models (like star formation and related feedback and chemical pollution by supernova). Further components like magnetic fields and high energy particles need additional modeling of their injection processes and evolution, and also have to be self consistently coupled with hydrodynamics. Here, cosmological simulations are a valuable probe of our understanding of the underlying physical processes and harbor enormous potential for the interpretation of observational data.

 
Cosmic Magnetism

Magnetic fields on large scales are observed within galaxies, along galactic outflows and even within the intra-cluster medium (ICM), filling the space inside galaxy clusters. It is expected that also the diffuse baryons within filaments and maybe even in voids -- which are the largest structures observed in the UniversCosmic Magnetisme -- are magnetized. However, despite their importance for various transport processes, our understanding of the origin, evolution and interplay of magnetic fields with structure formation is still very limited. Therefore, numerical experiments are a key tool for understanding the build up of large scale magnetic fields. They also enable the prediction of the expected properties of the magnetic field among different classes of objects (voids, filaments or galaxy clusters). Numerical simulations of the formation of cosmological structures and predictions of their magnetic field properties allow to put constraints on different models for the origin of cosmic magnetism. This also puts constraints onto the various mechanisms prop
osed for the magnetic seed fields, and relates the evolution of the magnetic field structure to the dynamical state of the cosmic structures. Particularly in case of 
galaxy clusters this allows to put additional constraints on the presence of cosmic ray protons by comparing in detail the induced radio emission in such simulations with observations. Additionally, such simulations can be used to constrain the transport and the deflection of ultra high energy particles (e.g. protons, photons) within the large scale structures of the universe.