The Local Group Mass in the light of Gaia

TL;DR

This paper refines the estimate of the Local Group's total mass using Gaia proper motions, correcting for biases like the LMC's influence, and finds a mass around 3.4 trillion solar masses, consistent with the combined mass of major members.

Contribution

It introduces a revised Timing Argument incorporating Gaia data and systematic corrections, providing a more accurate Local Group mass estimate.

Findings

Estimated Local Group mass: ~3.4 x 10^{12} M_sun

Correction factors reduce overestimation of mass

Total mass of major LG members is consistent with estimates

Abstract

High accuracy proper motions (PMs) of M31 and other Local Group satellites have now been provided by the {\it Gaia} satellite. We revisit the Timing Argument to compute the total mass $M$ of the Local Group from the orbit of the Milky Way and M31, allowing for the Cosmological Constant. We rectify for a systematic effect caused by the presence of the Large Magellanic Cloud (LMC). The interaction of the LMC with the Milky Way induces a motion towards the LMC. This contribution to the measured velocity of approach of the Milky Way and M31 must be removed. We allow for cosmic bias and scatter by extracting correction factors tailored to the accretion history of the Local Group. The distribution of correction factors is centered around $0.63$ with a scatter $\pm 0.2$, indicating that the Timing Argument significantly overestimates the true mass. Adjusting for all these effects, the estimated mass of the Local Group is $ M = 3.4^{+1.4}_{-1.1} \times 10^{12} M_{\odot}$ (68 % CL) when using the M31 tangential velocity $ 82^{+38}_{-35}$ km/s. Lower tangential velocity models with $59^{+42}_{-38}$ km/s (derived from the same PM data with a flat prior on the tangential velocity) lead to an estimated mass of $ M = 3.1^{+1.3}_{-1.0} \times 10^{12} M_{\odot}$ (68 % CL). By making an inventory of the total mass associated with the 4 most substantial LG members (the Milky Way, M31, M33 and the LMC), we estimate the known mass is in the range $3.7^{+0.5}_{-0.5} \times 10^{12} \, M_{\odot}$.