Clusters of Galaxies: Physics, Evolution and Cosmology

Clusters of galaxies - the largest bound structures in the universe - with their various constituents reveal more and more interesting processes and features. Especially the fields "physical processes in clusters", "cluster evolution" and the "use of clusters as diagnostic tools for cosmology" are of high interest. While the use of clusters for cosmological purposes has somewhat decreased recently, the physical processes in clusters obtain more attention than ever. In particular various interaction processes, metal enrichment and non-thermal processes are currently highly debated. Therefore we will study with this project the following four topics, which are accessible now by the large observing facilities in various wavelengths: (1) We will shed light on the mysterious Dark Matter and Dark Energy, which make up more than 90% of the universe, by determining cluster masses and by measuring the abundance of gravitational lensing in clusters. (2) The galaxies in clusters interact with the hot, thermal gas between them. There are various processes that cause galaxies to lose part of their gas, which lead to a transfer of metal-rich gas into the space between the galaxies. So far the efficiency and time dependence of these different processes are unclear. (3) As clusters are still in the process of formation merging processes are common phenomena. The merging of two clusters influences the properties of galaxies and of the hot gas between galaxies and it also can accelerate particles to relativistic energies. (4) The term "non-thermal phenomena" embraces all phenomena not connected to the thermal emission of the gas, e.g. radio synchrotron emission or magnetic fields. Determining and understanding the correlation between these phenomena in terms of physical processes will complete the study of the interactions in clusters of galaxies.

Clusters of galaxies the largest bound structures in the universe reveal more and more interesting processes. Clusters do not only consist of galaxies and dark matter, but there is also very hot gas (several million degrees) between the galaxies. When the galaxies move through the cluster, they feel this cluster gas. A pressure is acting on the galaxies, which causes the galaxies to lose their own gas. The galaxy gas is stripped off and is mixing with the cluster gas. Interestingly, the two different types of gas can be distinguished. Within the galaxies are many stars which produce elements like iron. Supernova explosions transport this iron from the interior of stars into the galaxy gas (also iron is in a gaseous state). The effect is that the iron concentration in the galaxy gas is much higher than in the cluster gas. Therefore one only has to measure the iron concentration to see where and how much gas has been stripped off the galaxies. The iron concentration can be measured by X-ray observations in the X-ray range because the cluster gas is so hot that it emits high energy radiation. The stripping process and gas motions can also be calculated by numerical simulations. In this project we have done both and have compared the results. Both methods show the same result: the two types of gas are mixing considerably less than expected. The iron distribution is both - in the observations and in the simulation - clumpy and not as homogenous as expected. This means, that single stripping processes are visible for a long time in the cluster. The comparison of the gas loss by stripping to the gas loss by other processes, e.g. internal processes in the galaxies like galactic winds, showed that stripping is very efficient. It is in particular efficient in the cluster centre, but also in the outskirts of clusters it is much more efficient than expected previously out to several million light years distance from the cluster centre. Gas is an important component of a galaxy, because from the gas the new stars are forming. Due to the stripping process a fraction of the gas is transported out of the galaxies. During this process the gas is compressed and therefore new stars are forming behind the galaxy. This is surprising as previously it was assumed that all the star formation takes place within the galaxies. Some of the stars formed in the wake are not bound to the galaxy they move away from the galaxy and form a stellar population between the galaxies. The other part of the stars fall back onto the galaxy and mess up the disc structure and the dynamics of the galaxy. Therefore a galaxy after a stripping process does not only contain less gas, but it looks also quite different.