Uncovering the first-infall history of the LMC through its dynamical impact in the Milky Way halo

The gravitational interactions between the LMC and the Milky Way can give rise to dynamical perturbations in the MW halo, leading to a biased distribution of stellar density and other kinematic signals. These disequilibrium phenomena exhibit variations under different parameter combinations of the MW-LMC model. In this work, we run 50 high-resolution N-body simulations spanning different masses and halo shapes of the Milky Way and LMC and investigate how the LMC-induced perturbations evolve with these model parameters. We measure the magnitude of kinematic perturbations from the mean velocities of simulated halo stars and identify a discontinuity between the first-infall and second-passage scenarios of the LMC's orbital history. We demonstrate that, due to the short dynamical times of the Galactic inner halo, the reduced perturbation magnitude in the second-passage scenario is mainly a result of the LMC's second infall into the MW, which starts at a much lower velocity relative to the inner halo compared to the first-infall scenario. Using a subset of ~ 1200 RR Lyrae stars located in the outer halo (50 ≤ RGC < 100 kpc), which are selected from a larger sample of 135,873 RR Lyrae stars with precise distance estimates from Gaia, we find the mean latitudinal velocity (vb) in the heliocentric frame to be vb = 30.8 ± 4.0 km/s. The observation contradicts the second-passage scenario and supports the first-infall scenario with a massive LMC (~ 2.1 × 10¹¹ M_) at infall, an oblate MW halo with a virial mass M₂₀₀ < 1.4 × 10¹² M_ and a flattening parameter $q > 0.7$.