Probing the asymmetry of matter distribution in galaxy clusters' environment with hydrodynamical simulations

Abstract: Galaxy clusters are the most massive cosmic structures in the Universe, and are connected at their peripheries to extended and complex filamentary structures, that act as funnels for the infalling material. This network of clusters and filaments is known as the cosmic web. For this reason, matter distribution in and around galaxy clusters is far from being isotropic, and the anisotropies are related to the processes of structure formation, with different matter components (dark matter, galaxies and different gas phases) being affected by different physical effects. Therefore, in this work we decided to study the asymmetries in the distributions of the different matter components from the central regions of galaxy to their outskirts. For this analysis we used the data from the cosmological hydrodynamical simulation IllustrisTNG. To characterize the asymmetries of the matter distribution we used two methods, both based on an harmonic decomposition of the projected matter density: the harmonic power excess, and the β parameters, which we introduce here for the first time. In the first part of the work we used the harmonic power excess to study the asymmetries of galaxy and gas distributions in and around galaxy clusters, with respect to the background distribution. We found that, on average, matter distribution in the inner cluster regions tends to be elliptical in shape, while the outskirts of clusters present a more complex harmonic signature, pointing towards the presence of filamentary structures. We also observed that different gas phases trace different structures in galaxy clusters: while the hot gas (that has been shock heated to high temperatures) is mainly found inside clusters, and traces their elliptical shapes, the warm-hot intergalactic medium is a better tracer for the filaments that connect to the clusters’ outskirts. In the second part of the project we used the β parameters to quantify the level of asymmetry in dark matter and gas distributions in clusters’ environments. We then related these quantities to the cluster properties. We found that the level of asymmetries is correlated to the geometrical properties of the matter density field, such as the ellipticity and the number of filaments around clusters, called the connectivity. It also shows dependencies on the radial distance from the cluster center, on the cluster mass, and on the cluster dynamical state. This work shows that the harmonic decomposition of the projected matter density can be a powerful tool to probe the departure from spherical symmetry of the matter distribution, and the level of asymmetry of the various matter components can be used as a probe of the cluster properties