The VPOS: a vast polar structure of satellite galaxies, globular clusters and streams around the Milky Way

Abstract: It has been known for a long time that the satellite galaxies of the Milky Way (MW) show a significant amount of phase-space correlation, they are distributed in a highly inclined Disc of Satellites (DoS). We have extended the previous studies on the DoS by analysing for the first time the orientations of streams of stars and gas, and the distributions of globular clusters within the halo of the MW. It is shown that the spatial distribution of MW globular clusters classified as young halo clusters (YH GC) is very similar to the DoS, while 7 of the 14 analysed streams align with the DoS. The probability to find the observed clustering of streams is only 0.3 per cent when assuming isotropy. The MW thus is surrounded by a vast polar structure (VPOS) of subsystems (satellite galaxies, globular clusters and streams), spreading from Galactocentric distances as small as 10 kpc out to 250 kpc. These findings demonstrate that a near-isotropic infall of cosmological sub-structure components onto the MW is essentially ruled out because a large number of infalling objects would have had to be highly correlated, to a degree not natural for dark matter sub-structures. The majority of satellites, streams and YH GCs had to be formed as a correlated population. This is possible in tidal tails consisting of material expelled from interacting galaxies. We discuss the tidal scenario for the formation of the VPOS, including successes and possible challenges. The potential consequences of the MW satellites being tidal dwarf galaxies are severe. If all the satellite galaxies and YH GCs have been formed in an encounter between the young MW and another gas-rich galaxy about 10-11 Gyr ago, then the MW does not have any luminous dark-matter substructures and the missing satellites problem becomes a catastrophic failure of the standard cosmological model.

• The paper demonstrates a strong alignment of satellite galaxies and young-halo globular clusters along a disc, with a chance alignment probability of under 0.1%.
• It introduces a novel method showing that half of the stellar streams align with the dominant orbital plane, contradicting isotropic infall models.
• The findings support the tidal dwarf galaxy hypothesis, suggesting that tidal forces from past interactions shaped the VPOS structure.

An Analysis of the Vast Polar Structure Surrounding the Milky Way

The study presents an examination of a vast polar structure (VPOS) encompassing the Milky Way (MW), which is characterized by a coherent arrangement of satellite galaxies, globular clusters, and stellar streams extending from the galactic halo inward to approximately 10 kpc from the Galactic center. This VPOS challenges the long-standing assumption that cosmological accretion can solely account for the distribution of such systems around the MW, and instead introduces the possibility of a tidal origin for these structures.

Key Findings and Methodology

1. Disc of Satellites (DoS) and Globular Clusters:
   • The paper utilizes a disc-fitting algorithm previously applied to MW satellite galaxies to assess the alignment of globular clusters in the MW halo.
   • A pronounced alignment is found between the young halo (YH) globular clusters and the DoS of satellite galaxies, suggesting that these clusters share a common origin with the satellites observed in the VPOS.
   • The probability of this alignment occurring by chance is less than 0.1 percent, underscoring the significance of these findings.
2. Analysis of Stellar Streams:
   • The study introduces a new method for determining the normal vectors of stream orbits around the MW, finding that half of the 14 streams analyzed align closely with the DoS normal.
   • This method reveals a preferred orbital plane for these streams that is not consistent with isotropic infall, lending further weight to the existence of the VPOS.
3. Comparison to Cosmological Accretion Models:
   • The VPOS's orientation and coherence challenge traditional cosmological models that predict independent, isotropically-distributed infall of dark matter subhalos.
   • Such models have been found inadequate in explaining the observed alignment without invoking a highly improbable degree of chance alignment.
4. Tidal Dwarf Galaxy (TDG) Hypothesis:
   • The paper explores the hypothesis that the components of the VPOS formed as TDGs through a past significant galactic encounter approximately 9 to 12 billion years ago.
   • Such a scenario accounts for the observed correlation in phase space and the range of objects included within the VPOS, supporting a scenario where tidal forces during a galactic interaction can create both dwarf galaxies and globular clusters within a coherent plane.

Theoretical and Observational Implications

The findings impel a reevaluation of a general assumption that cosmologically-driven processes dominate the formation and distribution of satellite systems. The possibility that these structures are a remnant of tidal interactions suggests alternative cosmological scenarios must be considered. In particular, the proposition that the MW might be seen as a relic polar-ring or polar-disc galaxy, formed from a major past interaction, invites novel lines of inquiry into the history and dynamics of the MW.

Future Directions

Future research should consider more robust modeling of TDG formation under various interaction conditions, particularly those relevant to the early universe, where gas-rich interactions would have been more common. High-resolution simulations could address the long-term stability of such systems and examine how subsequent dynamical interactions might shape them over cosmic time. Additionally, expanding the dataset of known MW halo objects and improving distance and proper motion measurements could refine constraints on the kinematic properties of the VPOS. Altogether, these studies would complement the current findings and provide further empirical and theoretical clarity on the genesis and evolution of VPOS-like structures.

By assessing the VPOS as potentially indicative of a significant formative event in the MW’s history, this study encourages a reevaluation of the processes which contribute to galaxy formation and evolution on cosmological scales.