Kinematic Signatures in the Stellar Halo from Cosmological Encounters between the Milky Way and its Clouds

Abstract

Recent theoretical and observational analysis of the interaction between the Milky Way (MW) and LMC suggest that it has a significant dynamical impact on the MW's stellar halo. We investigate this effect using simulations from the Latte project, a simulation suite from the Feedback In Realistic Environments 2 (FIRE-2) Project. By comparing simulations with and without an LMC-analog interaction, we show that fully cosmological LMC interactions create prominent velocity asymmetry in the stellar halo of the MW, resulting from both barycentric displacement (the "reflex motion") and the dynamical wake of the LMC. The strength and direction of this asymmetry depend on the mass ratio at pericenter and orbit of the LMC analog. We perform a spherical-harmonic decomposition of the velocities of halo star particles to confirm that the identified signatures are LMC-induced and persist even when LMC star particles are removed. We also show that this strategy separates and individually detects the dipole (l=1) of the global reflex motion and the quadrupole (l=2) of the local wake. These asymmetries are consistent with those identified in previous work using non-cosmological simulations; the dipole is easily distinguishable from other complex halo substructure using spherical harmonics while the quadrupole is sometimes confused. These findings support the detectability of MW--LMC interaction signatures in upcoming observational surveys of the MW stellar halo.