Boris Zhivkov Deshev will defend his PhD thesis "On the coevolution of galaxies and their host clusters" (physics) on 23 May 2019.
Prof. Alexis Finoguenov (University of Helsinki)
Prof. Peeter Tenjes (University of Tartu, Tartu obervatory)
Prof. Bodo Ziegler (University of Vienna)
Dr. Sean L. McGee (University of Birmingham)
Galaxies, are not uniformly distributed throughout space. Due to the continuous pull of gravity they form structures which range in densities from giant voids, almost completely devoid of galaxies, through sheets which surround the voids, filaments where sheets intersect and clusters which are forming at the crossing of filaments. Clusters of galaxies are often found in conglomerates called super-clusters which are the largest structures in the universe. This network of structures is continuously evolving with groups and smaller clusters of galaxies being accreted onto larger clusters. Sometimes large clusters also collide. It is observed that throughout this network the mixture of galaxies varies. The more rarefied environments are dominated by star forming, spiral galaxies while the interiors of large clusters are dominated by elliptical galaxies, devoid of star formation. The exact physical mechanisms that determines this connection between environment and morphology and activity of galaxies is not yet fully understood. In this thesis we search for a link between the evolution of galaxies and the evolution of the structures within which they are embedded. We analyse the star formation activity in galaxies which are members of a merging cluster of galaxies. We also analyse the cold gas content (fuel for star formation) of galaxies residing in a cluster with unusually high fraction of star forming galaxies. Finally we compare the galaxies found in these two clusters with a sample of other clusters at similar distance from us. We conclude that mergers of clusters of similar mass tend to quench star formation. We find indications that during the initial phase of the merger the star formation can be enhanced briefly. We find the gas content in cluster galaxies observed 2.5 Gyrs ago to be similar to that of local cluster-members, and that the increased fraction of star forming galaxies in distant clusters is likely related to processes working outside these clusters.