And Yet It Moves: The Dangers of Artificially Fixing the Milky Way Center of Mass in the Presence of a Massive Large Magellanic Cloud

Abstract

Motivated by recent studies suggesting that the Large Magellanic Cloud (LMC) could be significantly more massive than previously thought, we explore whether the approximation of an inertial Galactocentric reference frame is still valid in the presence of such a massive LMC. We find that previous estimates of the LMC's orbital period and apocentric distance derived assuming a fixed Milky Way are significantly shortened for models where the Milky Way is allowed to move freely in response to the gravitational pull of the LMC. Holding other parameters fixed, the fraction of models favoring first infall is reduced. Due to this interaction, the Milky Way center of mass within the inner 50 kpc can be significantly displaced in phase-space in a very short period of time that ranges from 0.3 to 0.5 Gyr by as much as 30 kpc and 75 km/s. Furthermore, we show that the gravitational pull of the LMC and response of the Milky Way are likely to significantly affect the orbit and phase space distribution of tidal debris from the Sagittarius dwarf galaxy (Sgr). Such effects are larger than previous estimates based on the torque of the LMC alone. As a result, Sgr deposits debris in regions of the sky that are not aligned with the present-day Sgr orbital plane. In addition, we find that properly accounting for the movement of the Milky Way around its common center of mass with the LMC significantly modifies the angular distance between apocenters and tilts its orbital pole, alleviating tensions between previous models and observations. While these models are preliminary in nature, they highlight the central importance of accounting for the mutual gravitational interaction between the MW and LMC when modeling the kinematics of objects in the Milky Way and Local Group.

The apostle project: Local Group kinematic mass constraints and simulation candidate selection

“…This choice is borne out of simplicity; correcting for the possible displacement caused by these massive satellites requires detailed assumptions about their orbits and their masses, which are fairly poorly constrained (see e.g. Gómez et al 2015).…”

Space Motions of the Dwarf Spheroidal Galaxies Draco and Sculptor Based on HST Proper Motions with a ∼10 yr Time Baseline

“…Instead, the MW moves in response to the gravitational influence of the satellites, particularly from the LMC (see Section 4.2), throughout the integration period (see also Gómez et al 2015).…”

“…We note that the orbital angular momentum vector of the LMC is roughly aligned with that of Draco. Gómez et al (2015) showed that the orbital barycenter of the MW-LMC system significantly changes over time, depending on the mass of the LMC. Therefore, as noted earlier, the MW is not held fixed in space, but rather moves in response to the force of the LMC as a function of time.…”

SMASH: Survey of the MAgellanic Stellar History

“…Such high velocities may allow for multiple interactions of the Clouds with the MW (Bekki 2011;Gómez et al 2015). A more likely scenario, however, is that the MCs are approaching the MW environment for the first time (Besla et al 2007;Gómez et al 2015), as favored by cosmological simulations (Boylan-Kolchin et al 2011;Busha et al 2011;Patel et al 2017). This discovery has forced a reinterpretation of many features of the Magellanic System, leading recent simulations (Besla et al 2010(Besla et al , 2012Diaz & Bekki 2012) to suggest that LMC-SMC interactions alone are responsible for the formation of the Magellanic Stream, including a potential direct collision between the LMC and SMC.…”