Planes of satellite galaxies

Sammanfattning: The Planes of Satellites Problem is an open question within the field of galaxy formation. It is based on the observations that the Milky Way's satellites, as well as those around the Andromeda (M31) galaxy, align in planes. This applies both to the satellites' positions lying close to the plane of best fit, and to the satellites' orbital planes aligning within a narrow angle. Such planar structures are not predicted by the standard model of cosmology, Lambda Cold Dark Matter (LCDM). Simulations based on this model only very rarely reproduce planes as thin and orbitally aligned as those we observe. In this project, we worked with five hydrodynamical simulations to study the planarity of each host-satellites system using both a spatial (plane height) and a kinematic metric (orbital dispersion). Two of the five simulations represent general cosmological simulations, while the remaining three form a set specifically modified to include a major merger of a given mass ratio. This allowed me to test whether the size of past mergers influence the formation of satellite planes. In all cases, we compared the five simulations to each other and the observed systems, as well as to isotropic realizations, both with randomized radial distributions and radial values corresponding to each simulation's satellite distribution. While the analysis focused on a single instance near z=0 from each simulation, we considered a longer time span in one simulation to obtain an impression of the longevity of a plane. From our results, we argue that all five simulations can be considered significantly planar, at least if the number of member galaxies belonging to the planes is kept variable. The small sample size makes it impossible to make generalizations concerning how common planes are in LCDM overall, but the presented cases certainly show that systems comparable to the Milky Way or M31 are possible within the standard theory. We further found a plane that was nearly static in both spatial and kinematic metrics across 341 Myr, although further analysis would be needed to establish how long-lived the plane is and how common such stable planes are in the LCDM framework. Lastly, the modified suite of simulations displayed a monotonic trend in which a larger major merger corresponded to a thinner and more orbitally aligned plane in the resulting system.