Impact of merger histories on the timing argument estimate of the Local Group mass

Abstract

The timing argument (TA) aims to find the total mass of the Local Group (LG) from the relative motions of the Milky Way (MW) and Andromeda Galaxy (M31). However, the classical TA always overestimates the LG mass, presumably because it does not account for the hierarchical scenario and other interactions such as that with the Large Magellanic Cloud (LMC). We focus on the impact of the recent major merger at M31 by using three merger models to find the peculiar motion of M31 within the simple two-body and point-mass scenario of TA. We found that the merger correction may affect the TA mass by either plus or minus 10-15% depending on the M31 tangential motion, which has very large uncertainties. If we consider a M31 merger configuration that reduces the TA mass by 10-15% to which we add the impact due to the LMC infall into the MW as reported in the literature, the TA mass would be found consistent with the LG mass from Hubble-Lemaitre flow. Galaxies are expected to experience about 16 major mergers each since z=11.5. Assuming all these mergers have similar impact on the TA mass as the most recent M31 merger, the ratio of LG mass to TA mass would be $0.85^{+0.65}_{-0.37}$ and such a TA mass is consistent with all the LG mass estimates. Our result also agrees with the findings using LG analogues in the cosmological simulations. We find that the TA mass estimate is limited by the hierarchical scenario, since it not possible to track the progenitors of both MW and M31 through so many mergers. We conclude that the MW-M31 dynamical system is far too complex to be modelled as a simple two-body point mass system.