Infoscience

Thesis

Large scale structures around medium mass galaxy clusters at intermediate redshift

Theory predicts that galaxies are not randomly distributed in the Universe. They form a complex network of filamentary structures, the Cosmic Web, divided in clusters, groups, filaments and walls. This picture has been confirmed by the large galaxy redshift surveys. Galaxy properties like morphology and star formation rates are strongly influenced by their environment. The most radical transformations are observed in galaxy clusters, located at the intersection of filaments. Nevertheless, the location where the environment starts to play a role and the main physical phenomenon responsible for these changes are still unknown. The aim of this thesis is to determine where and how galaxies are affected by the environment, at local and global scales. We gathered high quality photometry in the 5 optical bands $u$, $g$, $r$, $i$ and $z$ with MegaCam on the Canada-France-Hawaii Telescope for 3 medium mass galaxy clusters at intermediate redshift, with a spatial extent on the sky plane reaching 8 to 13 times the cluster virial radii. These clusters were selected to represent as much as possible progenitors of clusters in the local Universe. The core of these clusters have rich ancillary data from the ESO Distant Cluster Survey. The redshift and mass ranges of these clusters, combined to the wide field of the observations make this study unique. We computed photometric redshifts to map the galaxy distribution around the clusters. We were able to identify overdense regions forming groups and filaments, unveiling for the first time the large scale structures in such details for this type of clusters. We studied the fraction of galaxies in the red sequence in different environments, namely in the field, filaments and groups. We find that significant quenching processes are already at work in filaments, even at large cluster-centric distances. We also show that the local neighborhood of galaxies has a stronger impact on galaxy colors than the global environment. An optical spectroscopic follow-up with the VIsible MultiObject Spectrograph on the Very Large Telescope confirms our photometric results. In particular, the fraction of galaxies with detected \otwo emission line is smaller with increasing density. We also find that the star formation rate, determined from [OII], is in average weaker in groups and clusters than in lower density environments. This thesis opens new perspectives to study the way galaxies populate the Cosmic Web. Combined to weak-lensing analysis and cosmological simulations, it will allow a deeper understanding of the structure formation and their baryonic content. It is a first step before extending the results to much larger samples, with the help of the upcoming all-sky cosmological surveys such as Euclid or the Square Kilometer Array.

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