Stability of the Milky Way Satellite Galaxy Plane under the Influence of Neighbors

TL;DR

The paper addresses whether the Milky Way's thin plane of satellites can persist under external, time-varying perturbations in a nonspherical outer-halo potential. It employs two outer-halo setups in orbit integrations with AGAMA: (i) a fixed NFW monopole plus a quadrupole perturbation defined by amplitudes $a_{20}$ and $a_{22}$ with radial scaling $(r/kpc)^{3/2}$ up to $700$ kpc, and (ii) a time-dependent MW–M31 potential drawn from HESTIA Local Group analogs, analyzed over $5$–$6$ Gyr. The results identify a radius-dependent quadrupole amplitude threshold above which angular-momentum changes become large, and show that M31-driven torques can displace a planar configuration by more than $50$ kpc at outer radii over several Gyr, while inner regions remain comparatively stable over similar times. The work suggests the observed thin plane is likely a short-lived feature at large radii and underscores the need to account for environment and time dependence in reconstructing distant satellite and globular cluster orbits.

Abstract

Trajectories of test particles in a time-varying nonspherical gravitational potential model of our Galaxy are considered. The role of the quadrupole component of the potential, which at distances greater than 50 kpc is associated with the distribution of matter in the Galaxy's neighborhood (mainly with the influence of the galaxy M31), is studied. It is shown that perturbations of the potential created by the environment can significantly change the trajectories of particles at distances greater than 100 kpc from the Galactic center, but the magnitude of this effect depends on the still poorly known trajectory of the galaxy M31. For some variants of this trajectory, structures resembling a "thin plane" of satellite galaxies cannot exist for more than 2-3 billion years.

Figures (4)

• Figure 1: Histograms of the distribution of changes in angular momentum of particles initially uniformly distributed on spheres of radius 100, 150, and 200 kpc (solid, dash-dot, and dashed lines) for quadrupole component amplitudes $a_{20} = 0.5, 1.0,$ and $2.0$ (left, center, right).
• Figure 2: M31 trajectories in three model calculations. Solid line — 09_18, dashed — 17_11, dotted — 37_17. Top left: distance between M31 and the Galaxy (faint thick lines show Keplerian trajectories). Bottom left: angle between the direction to M31 from the Galactic center at present (t=0) and other times. Right: virial masses of the MW analog and M31 (top and bottom, respectively).
• Figure 3: Distances of test particles from the initial plane after 6 billion years in three models with different M31 trajectories (left to right), depending on the angle between the plane's normal and the direction to M31. Solid lines show median distances, shading shows distances from 10% to 90% percentiles, dashed line shows 50 kpc, the thickness of the "thin plane." Top row: initial radius 100 kpc, bottom row: 200 kpc.
• Figure 4: Median particle distance from the initial plane depending on the MW mass (left), initial orbit radius $r_0$ (center), and integration time (right). For left and right panels, $r_0=200$ kpc. Solid line — 09_18, dashed — 17_11, dotted — 37_17.