TCGCIEIS · The Cluster-Galaxy Connection: Interactions and Evolution

Final Activity Report Summary - TCGCIEIS (The Cluster-Galaxy Connection: Interactions and Evolution)

It is now common wisdom that structure in the Universe is formed hierarchically, by the merging of smaller mass units. Cosmological simulations have demonstrated in detail how large clusters of galaxies are assembled by the continuous accretion of small dark matter halos. During these violent merging events physical processes that have significant impact on the properties of the constituents of clusters, called intracluster medium (ICM) galaxies, take place and define their subsequent evolution. Regarding the effects on galaxies, it has been known for a long time that the their environment is responsible for their properties. The physical processes which stimulate or regulate star formation, nuclear activity and galaxies’ transformation are still debatable and it remains unclear which one and under which conditions dominates. The clusters are also affected, as the ICM, for example, is heated up galactic outflows which can be in the form of galactic winds from starburst galaxies and radio jets from active galaxies. During the hierarchical structure formation, we expect that the above processes take place and modify the properties of both clusters and galaxies with possible dramatic effects on their subsequent evolution. The aim of this project was to investigate the properties of clusters using X-ray observations to disclose the dynamical stage of the ICM, and the properties of their galaxies using either optical or infra-red data, to answer questions like: 1. Is there a relation between the cluster properties and the type of galaxies they host? 2. How is the ICM structure related to the properties of the galaxies? Initially, the study concentrated on a sample of nearby merging and evolving clusters. Using X-ray observations from the X-ray multi-mirror mission-Newton (XMM-Newton) satellite the hot ICM characteristics were found. Thanks to these studies, it was possible to put the clusters in a chronological order, being guided by their special X-ray characteristics. For example, one cluster was at early stages of merging, while another one was at later stages of its evolution, showing a temperature distribution expected in a cluster formed by the collision of two sub-clusters which had crossed each other. Using available optical observations of the above sample the distribution of different kinds of cluster galaxies was mapped. In one case, an increased amount of blue galaxies was found in regions in which the merging activity was high, as demonstrated by the X-ray images. The study of clusters that were close to ours ensured that data quality was such that the characteristic signatures of the physical processes taking place could be seen and studied in detail. The distributions of both X-ray and optical lights could be accurately mapped and compared. The shape of both distributions could be defined by simple, measurable parameters, such the location of the centre of the distribution and the orientation of an elongated distribution. These simple parameters could be determined even if the quality of the data did not allow for more in-depth investigations of the cluster properties. This was usually the case for clusters that were located at large distances. This idea was developed and applied to the nearby sample using the available X-ray and optical data. However, its application to clusters at higher redshifts was hampered by the unavailability of necessary data at the time.

Data: CORDIS, © European Union