The Planes of Satellite Galaxies Problem, Suggested Solutions, and Open Questions (1802.02579v1)

Abstract: Satellite galaxies of the Milky Way and of the Andromeda galaxy have been found to preferentially align in significantly flattened planes of satellite galaxies, and available velocity measurements are indicative of a preference of satellites in those structures to co-orbit. There is increasing evidence that such kinematically correlated satellite planes are also present around more distant hosts. Detailed comparisons show that similarly anisotropic phase-space distributions of sub-halos are exceedingly rare in cosmological simulations based on the $\Lambda$CDM paradigm. Analogs to the observed systems have frequencies of $\leq 0.5$ per cent in such simulations. In contrast to other small-scale problems, the satellite planes issue is not strongly affected by baryonic processes because the distribution of sub-halos on scales of hundreds of kpc is dominated by gravitational effects. This makes the satellite planes one of the most serious small-scale problem for $\Lambda$CDM. This review summarizes the observational evidence for planes of satellite galaxies in the Local Group and beyond, and provides an overview of how they compare to cosmological simulations. It also discusses scenarios which aim at explaining the coherence of satellite positions and orbits, and why they all are currently unable to satisfactorily resolve the issue.

Overview of the Planes of Satellite Galaxies Problem

The paper authored by Marcel S. Pawlowski provides a comprehensive review of the planes of satellite galaxies problem, which poses a significant challenge to the widely accepted $\Lambda$CDM cosmological model. Observations of satellite galaxies around the Milky Way and Andromeda reveal their alignment into planes that exhibit coherence in orbital motion, a phenomenon that appears exceedingly rare in $\Lambda$CDM-based simulations. This discrepancy is considered one of the most prominent small-scale problems of dark matter cosmology, largely unaffected by baryonic processes, as the distribution of sub-halos is governed primarily by gravitational dynamics on the relevant scales.

Observational Evidence

The existence of these planes is backed by substantial observational evidence from the Local Group and beyond. The Milky Way hosts the Vast Polar Structure (VPOS), wherein satellite galaxies align perpendicular to the galactic disk, with coherent orbits. The Great Plane of Andromeda (GPoA) is characterized by a similar spatial arrangement and possible rotational motion, featuring a significant kinematic correlation among satellites. Beyond the Local Group, the Centaurus A Satellite Plane (CASP) provides further evidence of such coherent structures.

Comparison with Cosmological Simulations

Attempts to model these structures within $\Lambda$CDM frameworks reveal these arrangements to be highly uncommon. The paper discusses detailed comparisons showing the low frequency of analogs in cosmological simulations, indicating that observed planes of satellites are statistical outliers. This rarity remains consistent in both dark-matter-only and hydrodynamical simulations, signifying a fundamental challenge to the $\Lambda$CDM model, which extends beyond the potential inaccuracies of baryonic physics modeling.

Suggested Solutions and Open Questions

Several scenarios have been proposed to explain the planar distribution and kinematic coherence of satellite galaxies:

1. Accretion along Cosmic Filaments: Filamentary accretion is an inherent trait in structure formation, yet appears insufficiently strong to produce the observed narrow planes.
2. Group Infall: The hypothesis that satellite galaxies were accreted as part of compact groups offers an explanation for their shared orbital characteristics, yet lacks support from the spatial extent of observed dwarf galaxy associations and numerical simulation results.
3. Baryonic Effects: While baryonic physics might alter the radial distribution of satellites, it does not directly impact their large-scale orbital coherence, leaving the problem unresolved beyond its influence.
4. Tidal Dwarf Galaxies (TDGs): Suggesting a profound shift to second-generation, dark-matter-free formations, TDGs provide a mechanism for forming correlated structures via tidal interactions. However, concerns regarding their metallicity and apparent dark matter content persist unless modeled under non-standard physics such as modified gravity.

Implications and Future Research Directions

This problem remains a significant puzzle with implications for our understanding of galaxy formation and the validity of dark matter-based cosmological models. It invites reconsideration of alternative theories, such as modified gravity models, and encourages deeper examination of environmental effects and historical galaxy interactions.

On an observational front, expanding surveys targeted at identifying coherent satellite systems in more distant galaxies will be essential. Efforts like the SAGA and DGSAT may provide critical data. Enhancing the resolution and scope of cosmological simulations will also refine our understanding. This research area promises to be at the forefront of cosmological exploration, not only challenging existing paradigms but potentially ushering in novel frameworks for interpreting cosmic phenomena.