The Effect of the Large Magellanic Cloud on the Dark Matter Halo of the Milky Way

Abstract

The Large Magellanic Cloud (LMC), the most massive satellite of the Milky Way(MW), is perturbing the dynamics and equilibrium of the MW. Understanding the nature and time evolution of these perturbations remains one of the most important challenges to the interpretation of 6D phase space measurements of tracers (satel- lites, streams, halo stars) of the dark matter (DM) distribution and assembly history of the MW. Using results from 8 high-resolution N-body simulations, I will describe the main perturbations caused by the LMC: the DM wake, the barycenter displace- ment, and the reflex motion. I show that these perturbations can be detected in both the kinematics and densities of the stellar halo beyond 50 kpc. Furthermore, I will illustrate how the present-day MW–LMC system can be modeled analytically using Basis Function Expansions. I maximize the physical signal in the representa- tion through a comprehensive noise analysis. As a result, the simulations of 10 8 DM particles representing the distorted MW(MW+LMC) system can be described by≈ 236(2067) coefficients. We find that the LMC induces asymmetric perturbations to the MW’s halo, which are inconsistent with oblate, prolate, or triaxial halos. The presence of the LMC also complicates efforts to recover the underlying shape of the MW’s halo. I also discuss how the LMC induces clustering of orbital poles of objects in orbit about the MW. By tracking the evolution of orbital poles for initially ran- dom, steady-state dark matter particles, I find that, after the infall of the LMC, the present-day orbital poles of particles , at Galactocentric distances beyond ≥50kpc, cluster near the present-day orbital pole of the LMC. This clustering can be up to a factor of ≈1.3 times higher than the density of orbital poles in an isolated MW halo and is most pronounced after the recent, close (≈50 Myr ago, 49 kpc) passage of the LMC. Given our location within the disk, particles in the simulation are observed from a non-inertial reference frame relative to the outer halo leading to an appar-16 ent alignment in their orbital poles. The DM wake induced by the LMC changes the kinematics of particles in the Southern Hemisphere. Observations of satellites selected within spatial planes also suffer from a bias, such that measuring orbital poles in a great circle in the sky enhances the probability of their orbital poles being clustered. I expect this scenario to be ubiquitous among hosts that have captured a massive satellite (at least ≈1:10 mass ratio) making a recent (≤ 1 Gyr) pericentric approach, where the massive satellite will cluster orbital poles of halo tracers.