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

Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía, 18080 Granada, Spain

^⋆ Corresponding author: ammartinez@iaa.csic.es

Received: 24 July 2025
Accepted: 4 September 2025

Abstract

Context. The Vast Polar Structure (VPOS) is a thin planar arrangement of co-orbiting dwarf galaxies that is nearly perpendicular to the Milky Way (MW) disc. Its origin and long-term evolution remain open questions.

Aims. We investigate the persistence and stability of the VPOS over time.

Methods. We identified VPOS member galaxies and integrated their orbits over the past 5 Gyr using accurate phase-space data and time-evolving gravitational potentials that accounted for the mutual interaction between the MW and the Large Magellanic Cloud (LMC).

Results. We identified 15 galaxies as members of the VPOS, including nine MW and six LMC satellites. The VPOS has remained a stable structure and maintained a roughly constant thickness (∼15 kpc), flattening (c/a ∼ 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 and only leads to mild perturbations. The structural properties of the VPOS remain almost unchanged, regardless of whether LMC satellites were included in the analysis. This indicates a smooth dynamical integration with the rest of VPOS members upon entering the virial radius of the MW. This minimal dynamical impact on the VPOS results from the remarkable alignment between the LMC orbit and the orientation of the plane.

Conclusions. The VPOS is not a transient alignment, but a long-lived planar structure in the MW system that has persisted for at least the last 5 Gyr. Notably, the VPOS predates the LMC infall, which rules out formation scenarios that are tied to its recent approach to the MW. Our findings suggest a strong connection between the VPOS and the LMC. This is consistent with a scenario in which the LMC is on its second pericentre and the VPOS originated primarily from satellites that were stripped during the first passage.