DOI: 10.5281/zenodo.7832266
title: The effect of the LMC on the Milky Way system

description: This repository contains final snapshots of N-body simulations of the Milky Way and LMC interaction, accompanying the review paper "The effect of the LMC on the Milky Way system" (Vasiliev 2023).<br> There are two models for the Milky Way - in both cases the disk and bulge are the same, while the halo is either spherical (and slightly heavier) or triaxial with a radially changing shape; the first one is an ad hoc but quite realistic model, and the second one is taken from the "Tango" paper (Vasiliev et al. 2021, MNRAS, 501, 2279) and provides a good fit for the Sagittarius stream, but in this repository it is rerun for 5 Gyr into the past instead of 3 Gyr in the original paper.<br> The LMC is a single-component spherical truncated NFW model with mass 5e10 or 15e10 (the second variant provides a better fit for many features in the Milky Way).<br> The initial conditions for the LMC orbit are adjusted so that it arrives to the right place at the right time (i.e., its present-day position and velocity match observations to within 1 kpc and a couple km/s), but the choice of its current phase-space coordinates differs between the older Tango simulation and the more recent simplified spherical halo simulation.<br> The files "snapshot.npz" in each folder contain the present-day snapshots in Galactocentric coordinates: the Sun is located at -8.2,0,0 and the LMC sits around -0.5,-41,-27 kpc. "posvel" is the Nx6 array of positions and velocities of particles and "mass" is the array of<br> particle masses, in the units of 1 kpc, 1 km/s, 1 Msun; the time unit is close to 1 Gyr. The first 1e6 particles are the LMC, the next 1e6 particles are the stellar disk and bulge of the Milky Way, and the remaining 4e6 particles are its dark halo.<br> Other snapshots in the simulation are not provided, but instead the time-dependent potential of the entire system is represented in the format of the Agama stellar-dynamical framework (https://agama.software). The total potential is given in the non-inertial reference frame centered on the Milky Way center, and consists of three components: the Milky Way itself, the moving LMC (its trajectory in the Galactocentric coordinates is stored in trajlmc.txt), and the spatially uniform but time-dependent acceleration associated with this non-inertial frame.<br> There are two versions of the potential in each directory: "frozen" retains the initial potential of both galaxies (i.e. they are non-deforming, though of course the LMC is moving in space), and "evolving" contains potentials extracted from the actual N-body snapshots in the last 2 Gyr, represented by multipole expansions (one for the entire LMC, the other is for the Milky Way halo, while its disk+bulge are assumed to be fixed). The latter variant is more accurate and tracks the deformations of both galaxies, but is more expensive when used for orbit integrations.<br> The example Python script illustrates the usage of these potentials and reproduces one of the figures from the paper: the kinematic perturbations in the Milky Way halo at present day, which disappear when the orbits of stars are rewound back in time in the provided time-dependent potentials, starting from the current phase-space coordinates of particles.