The coherent motion of Cen A dwarf satellite galaxies remains a challenge for $Λ$CDM cosmology (2012.08138v2)

Abstract: The plane-of-satellites problem is one of the most severe small-scale challenges for the standard $\Lambda$CDM cosmological model: several dwarf galaxies around the Milky Way and Andromeda co-orbit in thin, planar structures. A similar case has been identified around the nearby elliptical galaxy Centaurus A (Cen A). In this Letter, we study the satellite system of Cen A adding twelve new galaxies with line-of-sight velocities from VLT/MUSE observations. We find 21 out of 28 dwarf galaxies with measured velocities share a coherent motion. Similarly flattened and coherently moving structures are found only in 0.2% of Cen A analogs in the Illustris-TNG100 cosmological simulation, independently of whether we use its dark-matter-only or hydrodynamical run. These analogs are not co-orbiting, and arise only by chance projection, thus they are short-lived structures in such simulations. Our findings indicate that the observed co-rotating planes of satellites are a persistent challenge for $\Lambda$CDM, which is largely independent from baryon physics.

• The paper identifies a co-moving planar structure in 21 out of 28 Cen A dwarf satellites using new VLT/MUSE velocity data.
• The analysis finds that such coherent structures occur in only 0.2% of ΛCDM simulations, highlighting a significant model discrepancy.
• The study suggests that the observed alignment challenges the ΛCDM paradigm, warranting further observational and simulation efforts.

Overview of the Coherent Motion of Cen\,A Dwarf Satellite Galaxies and Challenges to $\Lambda$CDM Cosmology

The paper investigates the coherent motion of dwarf satellite galaxies around Centaurus A (Cen\,A) and assesses its implications for the standard $\Lambda$ Cold Dark Matter ($\Lambda$CDM) model of cosmology. The paper extends previous research by incorporating new velocity data for twelve additional dwarf galaxies, utilizing observations from the Very Large Telescope’s Multi Unit Spectroscopic Explorer (VLT/MUSE). The authors argue that the observed alignment and motion of these galaxies present a significant challenge for the $\Lambda$CDM paradigm, as similar configurations are notably rare in cosmological simulations.

Key Findings

1. Co-Moving Plane of Satellites:
   • Out of 28 observed dwarf satellites of Cen\,A, 21 exhibit coherent motion within a flattened structure. Such configurations mirror planar satellite systems previously observed around the Milky Way and Andromeda, where dwarf galaxies also co-orbit in a planar fashion.
2. Frequency in Simulations:
   • Analysis using the Illustris-TNG100 cosmological simulation indicates that Cen\,A analogs displaying similar coherent structures are remarkably rare, occurring in only about 0.2% of cases. The phenomenon holds across both dark-matter-only and hydrodynamical simulations, suggesting that baryonic processes do not alleviate the discrepancy.
3. Statistical Significance:
   • The statistical probability of the observed coherence occurring by chance is calculated to be low, reinforcing the suggestion of an underlying physical phenomenon.

Implications and Future Directions

The findings underscore a persistent challenge to $\Lambda$CDM cosmology, which traditionally predicts more isotropic satellite distributions around host galaxies. The coherent planar structures observed in the Centaurus group contribute to a broader discourse on the discrepancies between observed satellite dynamics and the expectations derived from $\Lambda$CDM simulations.

Cosmological Significance:

• The rarity of such coherent structures in simulations highlights potential gaps in the standard cosmological model. These gaps may necessitate revisions in the understanding of dark matter interactions or alternative models that account for the observed anisotropies.

Prospects for Further Investigation:

• Additional observational campaigns could help ascertain if similar structures exist around other galaxies, thereby assessing whether these coherent satellite alignments are indeed statistical anomalies or indicative of new physics. Future high-resolution simulations might focus on exploring conditions or initial setups that could give rise to such kinematic coherence.

Concluding Remarks

This paper reinforces the need for continuous scrutiny of small-scale structures within the cosmological framework, particularly as new observational data provides opportunities to test and refine theoretical models. While the $\Lambda$CDM model effectively describes large-scale structures, phenomena such as the planes of co-moving dwarf satellites challenge its applicability on smaller scales, potentially leading to significant theoretical advancements.