Co-orbiting satellite galaxy structures are still in conflict with the distribution of primordial dwarf galaxies

Abstract: Both major galaxies in the Local Group host planar distributions of co-orbiting satellite galaxies, the Vast Polar Structure (VPOS) of the Milky Way and the Great Plane of Andromeda (GPoA). The $\Lambda$CDM cosmological model did not predict these features. However, according to three recent studies the properties of the GPoA and the flattening of the VPOS are common features among sub-halo based $\Lambda$CDM satellite systems, and the GPoA can be naturally explained by satellites being acquired along cold gas streams. We point out some methodological issues in these studies: either the selection of model satellites is different from that of the observed ones, or an incomplete set of observational constraints has been considered, or the observed satellite distribution is inconsistent with basic assumptions. Once these issues have been addressed, the conclusions are different: features like the VPOS and GPoA are very rare (each with probability $\lesssim 10^{-3}$, and combined probability $< 10^{-5}$) if satellites are selected from a $\Lambda$CDM simulation combined with semi-analytic modelling, and accretion along cold streams is no natural explanation of the GPoA. The origin of planar dwarf galaxy structures remains unexplained in the standard paradigm of galaxy formation.

Analysis of Co-orbiting Satellite Galaxy Structures in the Local Group

The paper by Pawlowski et al. provides an in-depth analysis of co-orbiting satellite galaxy structures within the context of the $\Lambda$CDM cosmological model, specifically focusing on the Vast Polar Structure (VPOS) of the Milky Way and the Great Plane of Andromeda (GPoA). These planar satellite distributions represent significant challenges to the standard cosmological framework and have prompted extensive study due to their unexpected configurations.

The authors critically examine recent claims that $\Lambda$CDM simulations can account for these planar structures. Notably, three studies have suggested that the characteristics of the GPoA and the VPOS are not uncommon in simulated satellite systems, attributing their formation to the accretion of satellites along cold gas streams. Pawlowski et al. contest these conclusions, identifying methodological issues in the selection and analysis of model satellites when compared to observed systems. They argue that the methodologies in these studies either select model satellites differently than the observational criteria or fail to integrate the complete set of observational constraints.

The study utilizes data from the Millennium II simulation, applying stringent criteria to identify GPoA and VPOS analogs. Their analysis suggests that planar satellite distributions similar to the observed GPoA and VPOS are exceedingly rare in $\Lambda$CDM-based simulations. For instance, they estimate a probability of less than $10^{-5}$ for the simultaneous occurrence of structures analogous to the VPOS and GPoA, questioning the validity of these features as natural products of the $\Lambda$CDM universe.

Furthermore, the paper scrutinizes the hypothesis that the GPoA can be explained by accretion along cold streams, as previously posited. By modeling streams as orbital planes rather than radial lines, the authors demonstrate that it is improbably rare for satellite planes to coalesce in such a narrow and coherent manner as observed in the GPoA.

In terms of implications, the authors argue that their findings pose a substantial challenge to the $\Lambda$CDM paradigm, suggesting that either the understanding of satellite dwarf galaxy formation or the foundational cosmological model may require revision. The results emphasize the importance of incorporating dynamic and kinematic information into cosmological simulations to capture the high-dimensional nature of satellite configurations.

The paper's conclusions prompt further exploration into alternative models or mechanisms that might account for these observations, including modified theories of gravity or alternative dark matter scenarios. Future studies are encouraged to refine semi-analytic models and consider higher-resolution simulations to better explore the complexities of these satellite galaxy structures.

Overall, the research by Pawlowski et al. calls into question the adequacy of current $\Lambda$CDM models in explaining prominent satellite galaxy structures and underscores the need for continued investigation into both theoretical models and observational strategies to resolve these cosmic puzzles.