Constraining the Milky Way dark matter halo with LMC-induced reflex motion

TL;DR

This study investigates how the Large Magellanic Cloud's infall influences the Milky Way's halo, revealing that the reflex motion's direction and halo contraction are sensitive to the outer dark matter profile, but its amplitude alone offers limited constraints.

Contribution

It demonstrates the dependence of the Milky Way-LMC reflex motion on the outer dark matter halo profile using N-body simulations, highlighting limitations in constraining halo parameters from amplitude alone.

Findings

Reflex motion amplitude varies with radius but is insensitive to outer halo slope.

Direction of disc motion is highly sensitive to outer halo density distribution.

Halo contraction and oscillation frequency depend strongly on the outer DM profile.

Abstract

Modelling perturbations of the Milky Way (MW) halo induced by the infall of the Large Magellanic Cloud (LMC) offers new avenues to constrain the dark matter (DM) distribution in our Galaxy. A key observable is the reflex motion of the MW disc with respect to the halo induced by the LMC's infall, which imprints a velocity dipole on kinematics of halo stars. Here we investigate how the dipole varies with Galactocentric radius, and study the sensitivity of the reflex motion signal to different DM outer-halo profiles. Using a suite of basis function expansion (BFE) simulations with truncated NFW profiles ($\rho \propto r^{-\beta}$ beyond $r=50$ kpc), our $N$-body models show that (i) The reflex motion amplitude varies with Galactocentric radius but is largely insensitive to the outer DM slope, implying that the MW-LMC mass ratio alone does not set the dipole strength. (ii) In contrast, the direction of the disc motion is very sensitive to the density distribution of the outer DM halo. (iii) The contraction of the MW halo induced by the LMC's gravitational pull also depends strongly on the outer DM halo profile. (iv) We find a halo instability whose oscillation frequency increases with $\beta$ producing a potentially observable signature - a sinusoidal pattern of the mean radial velocity of halo stars. Finally, using BFE coefficients we find that steeper truncations produce smaller dipole distortions, while amplifying the quadrupole distortion. These results highlight the limited constraining power of the reflex motion amplitude alone for outer MW profile parameters.