Planes of satellite galaxies

Abstract

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.

Popular Abstract

The planes of satellite galaxies problem is a hotly debated topic in the research field of galaxy formation. This problem is concerned with small, so-called dwarf galaxies. They often contain only a few thousand stars and are 10 to 1000 times smaller than the Milky Way, but they are much more common than large spiral galaxies. The Milky Way is surrounded by at least 60 such galaxies, orbiting their large host as satellites. A similar system is seen around the nearby Andromeda (M31) galaxy, which has at least 27 satellites.

The interesting detail about this host-satellite system is that many of the satellites in each system align in a plane. At fist glance, this might not come as a surprise: The planets in the solar system all orbit in a single plane, and even the Milky Way spiral arms lie within the thin galactic disk. However, these two examples can be explained with our current understanding of physics – the planes of satellites cannot. The standard model of cosmology does not explicitly predict such planes to form, and simulations that were developed to recreate similar systems only rarely produce thin planes.

This tension between theory and observation has created a vivid debate around the topic of planes of satellite galaxies. Some claim that it is a failure of our standard model, suggesting instead theories like "Modified Newtonian Gravity" to replace the assumption of dark matter. Others suggest that the observed systems may have formed this way due to specific factors, like several satellites joining as a group. With limited evidence for any given theory, the question of why planes of satellites form remains unanswered.

This project works with two different sets of simulations to explore the discrepancy between our theory’s predictions and the observed reality. One pair of simulations provides a general impression of the results from cosmological simulations. The other set of three simulations is modified to include specific formation events in which the host galaxy merges with another galaxy. This allows us to explore large mergers as a possible explanation for the formation of planes.

• author: Kocher, Jessica Laura Erika
• supervisor: Oscar Agertz
• organization: Lund Observatory - Has been reorganised
• course: ASTK02 20212
• year: 2022
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• report number: 2022-EXA188
• language: English
• id: 9075753
• date added to LUP: 2022-08-15 15:02:36
• date last changed: 2022-08-15 15:02:36