Implications of the travel velocity of the Milky Way on Local Group mass estimates from the Timing Argument

The total mass of the Local Group (LG) is a fundamental quantity that enables interpretation of the orbits of its constituent galaxies and is needed to place the LG in a cosmological context. However, measuring the LG mass is not straightforward, as the distribution and quantity of dark matter is unknown. One method of determining the LG mass is the Timing Argument (TA), which models the dynamics of the MW—M31 system as a two-body (Keplerian) orbit, and which depends strongly on the measured kinematics of M31. Previous TA studies of the LG mass have attempted to correct for the impact of the LMC on the inferred mass, but suffer from unknowns in the mass and dynamical modeling of the LMC interaction with the MW. However, recent studies of tracers in the MW stellar halo have found that the MW disk is moving with a lower bound “travel velocity” of 32±4 km/s with respect to its outer halo, a byproduct of its merger history and the recent pericentric passage of the LMC. This novel measurement allows us to place model-independent, empirical constraints on the LG mass that account for the measured reflex motion of the disk for the first time. In this talk, I will present our TA model that incorporates the travel velocity of the MW disk using several different compilations of recent kinematic measurements of M31. I will show that we recover lower LG masses than past TA results, and measure a total mass of ~(4.5 ± 0.7) × 10¹² Msun that is consistent between datasets. Additionally, I will explain how measurements of more distant tracers may yield even larger values for the travel velocity, which would further decrease the inferred LG mass. As a result, the newly measured travel velocity directly implies a lower LG mass than from a system with a static MW halo, and must be considered in future dynamical studies of the Local Volume.