Evolutionary Processes in Interacting Clusters of Galaxies

We propose to study galaxy clusters, their formation and the physical processes driving their evolution. In the hierarchical model of structure formation, clusters of galaxies form by accretion and merging of smaller sub- clusters and groups. Major cluster-cluster collisions are the most energetic event since the Big-Bang. Therefore such merging events can strongly affect the physical properties of the different cluster components, i.e. dark matter, galaxies, gas, relativistic particles and magnetic fields. Up to now the effects of mergers on: a) the thermal and non-thermal components of the intra-cluster medium (gas and relativistic particles), and b) the internal dynamics of clusters have been analysed in detail. Currently, the increasing spatial and spectral resolution achieved both from the numerical and from the observational point of view opens up the possibility to study also the effects of mergers on the evolution of cluster galaxies. This gives us the opportunity to answer one of the main open questions of modern cosmology: the origin of the observed evolution in the star-formation rate (SFR) of cluster galaxies. Different physical mechanisms (e.g. ram-pressure stripping and/or compression of the inter-stellar medium (ISM), galaxy-galaxy interactions and mergers) have been proposed to drive the observed change in galaxy properties. Due to the high energies involved in cluster-cluster collisions, the merging event can enhance the efficiency of these mechanisms. Merging clusters are therefore privileged laboratories to test the physics of evolutionary effects. In this picture, our main scientific goal is to determine how the merging event affects the SFR of cluster galaxies. Our two level approach consists of a) observing in optical, X-ray, radio and IR a sample of interacting clusters, and b) tracing by N-body and hydrodynamic simulations the evolution of collisionless matter (i.e. galaxies and dark matter) and of intra-cluster gas, and their interactions. By comparing multi-wavelength observations to numerical simulations, we will be able to determine the evolutionary stage and the collision parameters of merging clusters. The next steps will be to estimate the SFR of cluster members through our multi-wavelength observations. We can therefore determine if the net effect of the merging process is an increase or a decrease the SFR of cluster galaxies. Finally, the comparison between our observational results and detailed numerical simulations will allow us to investigate the physical mechanisms driving the evolution in the SFR of cluster galaxies. As clusters of galaxies are the largest bound structures in the Universe, a clear and complete picture of their evolutionary proceses will provide fundamental clues on cosmological models and structure formation.

Galaxies are not at rest in the universe, but they move with high velocities. In particular the galaxies that reside in large accumulations of galaxies - the so-called clusters of galaxies - move with velocities of several hundred km/s. Also the clusters of galaxies have velocities and sometimes they collide with each other, which results eventually in merging. These merging processes are the most energetic processes since the Big Bang. All this dynamics influences also the galaxies. The space between the galaxies is not empty, but there is gas, the so-called intra- cluster gas. This intra-cluster gas has very low densities, but the galaxies feel it nevertheless as some kind of headwind. Hence the intra-cluster gas exerts some pressure onto the gas within the galaxies and therefore it increases the density of the gas in the galaxies. New stars form from gas, and especially many stars form in regions, where the gas is dense. For this reason all the above described dynamics have the effect that many more stars form in galaxies which are located in a merging cluster of galaxies. New stars are hot and bright, so that the effect can be detected in observations. A second effect of the "headwind" is that it strips off gas from the galaxy. This stripped gas behind the galaxies is also compressed by the intra-cluster gas and hence also here new stars are formed. This new finding means, that stars can also form in the intergalactic space and a population of intergalactic stars is produced.