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

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 Milky Way 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.