The long-term stability of the Vast Polar Structure and its connection to a possible previous passage of the LMC

TL;DR

This study interrogates the long-term stability of the Vast Polar Structure (VPOS) around the Milky Way by backward-integrating the orbits of 58 dwarf galaxies over 5 Gyr within six time-evolving MW–LMC potentials. Using Monte Carlo sampling of 6D phase-space data and PCA-based plane diagnostics, the authors identify 15 VPOS members and show that the VPOS remains a thin, coherently rotating plane with $c/a\sim0.2$ and thickness about $15\,$kpc across all models, with only mild perturbations from the LMC. They demonstrate that the VPOS predates the LMC’s recent infall, arguing against a formation scenario tied to the LMC’s current approach, and find a strong dynamical link between the VPOS and the LMC, potentially implying the VPOS originated from satellites stripped during an earlier LMC pericentre. While supporting a close association with the LMC, the results acknowledge a complex origin including group accretion and occasional rearrangements (e.g., Grus II), and call for future Gaia improvements and high-resolution simulations to refine the VPOS’s origin and broader implications for planes of satellites in the Local Group.

Abstract

The Vast Polar Structure (VPOS) is a thin, planar arrangement of co-orbiting dwarf galaxies, nearly perpendicular to the Milky Way (MW) disc. In this work we investigate the persistence and stability of the VPOS over time. We identify VPOS member galaxies and integrate their orbits over the past 5 Gyr using time evolving gravitational potentials that account for the mutual interaction between the MW and the Large Magellanic Cloud (LMC). The reconstructed trajectories allow us to examine the long-term evolution of the VPOS. We identify 15 galaxies as members of the VPOS, including 9 MW and 6 LMC satellites. We find that the VPOS has remained a stable structure, maintaining a roughly constant thickness ($\sim$ 15 kpc), flattening ($c/a \sim$ 0.2), and orientation over time. While the LMC exerts a strong gravitational influence on the MW satellites, its impact on the VPOS is limited, leading only to mild perturbations. The structural properties of the VPOS remain almost unchanged, whether or not LMC satellites are included in the analysis, indicating a smooth dynamical integration with the rest of VPOS members upon entering the MW virial radius. This minimal dynamical impact on the VPOS results from the remarkable alignment between the LMC's orbit and the plane's orientation. We conclude that the VPOS is a stable, long-lived structure that predates the recent infall of the LMC and retains nearly constant structural properties over the last 5 Gyrs. Our findings suggest a strong connection between the VPOS and the LMC, consistent with a scenario in which the LMC is on its second pericentre and the VPOS originated primarily from satellites stripped during the first passage.

Figures (6)

• Figure 1: Distribution of the orbital poles for 58 dwarf galaxies around the MW in Galactic coordinates. Solid points represent the orbital poles from the MC realizations of each galaxy. The color of the points indicates VPOS membership: orange for on-plane galaxies, blue for off-plane, and grey for uncertain membership. Empty circles mark the median orbital pole of each galaxy. Green crosses denote the VPOS normal vector direction as reported in PawlowskiKroupa2013, green '+' marker denotes the opposite direction, while green lines outline the region covering 10% of the sky around them. Labels are abbreviated versions of the galaxy names. Each plot shows only the poles of galaxies within a specific Galactocentric distance range to ease visualization.
• Figure 2: Spatial distribution and orbital histories of VPOS member galaxies. The panels show the positions of on-plane galaxies and their past trajectories under potential V23, at the present time (left) and at $t = -1$ Gyr (right). The coordinate system used ($x', y', z'$) is not Galactocentric, but instead defined by the principal axes of the galaxy distribution at $t = 0$; in this frame, the $z'$-axis is aligned with the normal vector of the VPOS. Points indicate galaxy positions at the corresponding time, while lines trace their past trajectories. Red denotes MW satellites, blue indicates LMC satellites, and black marks the LMC itself. Carina II and Horologium II are shown in green. Although not formally classified as on-plane members, they are LMC satellites and thus likely part of the VPOS (see Section \ref{['sec:members']}). Dashed lines are used to represent the trajectories of Bootes III and Sculptor, the two counter-rotating galaxies of the VPOS. A grey annulus at the origin represents the MW stellar disk for reference, with a radius of 15 kpc.
• Figure 3: Time evolution of the structural parameters of the VPOS measured from the sample of on-plane MW satellites only. Each row of panels represents the parameters for a different gravitational potential. Columns of plots represent from left to right: (1) the short-to-long axis ratio, $c/a$ (solid line), and intermediate-to-long axis ratio, $b/a$ (dashed line), (2) the thickness of the VPOS measured with the RMS, (3) the thickness of the VPOS measured with the MAD, (4) the Galactic longitude ($l$, dashed line) and latitude ($b$, solid line) of the direction of the normal vector of the VPOS, and (5) the MRL of the normal vector, all of them as a function of time. Lines represent the median values of the different metrics, and shaded areas the 16th and 84th quantiles.
• Figure 4: Evolution of the orientation of the VPOS. Color coded curve represents the time evolution of the direction of the normal vector of the VPOS in Galactic coordinates, measured from the on-plane MW satellites under the potential V23 (see Section \ref{['sec:VPOSmw']}). Green cross represents the VPOS normal direction reported in PawlowskiKroupa2013 and green circle the border of the area around it encompassing 10% of the sky.
• Figure 5: Time evolution of the structural parameters of the VPOS measured from the MW on-plane satellites only sample, and the MW and LMC on-plane satellites sample. The distribution of plots and markers coincides with Fig. \ref{['fig:VPOSevolutionMW']}, however the analysed time span is shorter, starting when the LMC gets within 250 kpc from the MW (see Section \ref{['sec:vposevo']}). Red lines and shaded areas represent the parameters of the VPOS measured with the MW+LMC on-plane satellite sample, blues one are for the MW sample only. Dashed vertical lines show the time at which the LMC performed its pericentre.