The Milky Way's stellar streams and globular clusters do not align in a Vast Polar Structure

Abstract: There is increasing evidence that a substantial fraction of Milky Way satellite galaxies align in a rotationally-supported plane of satellites, a rare configuration in cosmological simulations of galaxy formation. It has been suggested that other Milky Way substructures (namely young halo globular clusters and stellar/gaseous streams) similarly tend to align with this plane, accordingly dubbed the Vast Polar Structure (VPOS). Using systemic proper motions inferred from Gaia data, we find that globular cluster orbital poles are not clustered in the VPOS direction, though the population with the highest VPOS membership fraction is the young halo clusters (~30%). We additionally provide a current census of stellar streams, including new streams discovered using the Dark Energy Survey and Gaia datasets, and find that stellar stream normals are also not clustered in the direction of the VPOS normal. We also find that, based on orbit modeling, there is a likely association between NGC 3201 and the Gj\"{o}ll stellar stream and that, based on its orbital pole, NGC 4147 is likely not a Sagittarius globular cluster. That the Milky Way's accreted globular clusters and streams do not align in the same planar configuration as its satellites suggests that the plane of satellites is either a particularly stable orbital configuration or a population of recently accreted satellites. Neither of these explanations is particularly likely in light of other recent studies, leaving the plane of satellites problem as one of the more consequential open problems in galaxy formation and cosmology.

• The paper analyzes Gaia data, finding that Milky Way stellar streams and globular clusters do not generally align with the Vast Polar Structure (VPOS).
• Analysis shows only around 30% of young halo globular clusters align with the VPOS direction, and stellar stream normals are distributed more isotropically than planarly.
• Despite lack of general alignment, the study identifies potential specific associations between individual globular clusters and streams, such as NGC 3201 and the Gjöll stream.

The Alignment of Milky Way's Substructures with the Vast Polar Structure

The paper "The Milky Way's stellar streams and globular clusters do not align in a Vast Polar Structure" provides a critical reevaluation of the spatial alignment of various substructures within the Milky Way galaxy. The study specifically investigates whether globular clusters and stellar streams align with the previously suggested Vast Polar Structure (VPOS), which is a planar configuration proposed to include the Milky Way’s satellite galaxies.

Summary of Key Findings

The researchers utilize systemic proper motions derived from Gaia data to analyze the orbital poles of globular clusters and the normal vectors of stellar streams. Their investigation leads to several significant findings:

1. Globular Clusters Misalignment: The analysis reveals that globular cluster orbital poles do not show clustering in the VPOS direction. Particularly, only about 30% of young halo globular clusters align with the VPOS. This finding challenges the earlier hypothesis that globular clusters contribute significantly to the VPOS.
2. Stellar Streams Distribution: A comprehensive census of stellar streams, including discoveries from the Dark Energy Survey and Gaia data, shows that stream normals are not predominantly clustered in the VPOS direction. This hints toward an isotropic distribution rather than a preferential planar arrangement.
3. Potential Specific Associations: Although a general alignment with the VPOS is not found, specific associations between individual globular clusters and streams are identified using orbital modeling. Notably, a potential association between NGC 3201 and the Gjöll stellar stream is suggested, highlighting intriguing dynamical interactions in the galaxy.

Implications of the Research

The findings underscore the complexity of the Milky Way’s structural formation and suggest the necessity of revisiting the VPOS paradigm for a more comprehensive understanding. The lack of alignment might imply that the VPOS, previously believed to include broader substructure elements such as streams and clusters, is a more isolated phenomenon pertaining mainly to satellite galaxies.

The results also pose intriguing questions regarding the stability and origin of the VPOS. Potential explanations such as the stability of polar orbital configurations and recent group infall scenarios are explored, considering the planes' observed longevity and coherence in some galaxies contrasted with simulations and model predictions.

Additionally, the study reinforces the importance of detailed kinematic and dynamical studies in understanding the accretion history and structural evolution of the Milky Way. The methodologies employed, especially the use of Gaia data for proper motion analysis, demonstrate the power of modern astrometric surveys in uncovering the underlying dynamics of galactic structures.

Speculative Insights and Future Directions

The paper opens several avenues for future research and investigation. Firstly, there is a need to explore the potential role of large-scale galactic interactions and the gravitational influence of massive satellites like the Large Magellanic Cloud in shaping the observed distributions. Moreover, incorporating baryonic physics in cosmological simulations may also shed light on the current discrepancies between observation and theory.

Finally, the study suggests the exploration of non-linear dynamics and chaotic mixing processes that could lead to the dissolution of such coherent structures over time, enriching the methodological toolkit for galactic archaeology. By resolving these complexities, future work can better contextualize the Milky Way’s formation within the broader narrative of cosmic structure evolution.