20-24 March 2023
Haus H, Telegrafenberg
Europe/Berlin timezone

The Mass of the Local Group

21 Mar 2023, 14:45
Haus H, Telegrafenberg

Haus H, Telegrafenberg

Potsdam, Germany
Invited topical talk SESSION 2 : Sizes, Masses, and Formation histories of the Milky Way and of Andromeda SESSION 2 : Sizes, Masses, and Formation histories of the Milky Way and of Andromeda


Wyn Evans (Cambridge University)


We review estimates of the total mass of the Local Group. High-accuracy proper motions (PMs) of M31 and other Local Group (LG) satellites have now been provided by the Gaia satellite. We revisit the timing argument to compute the total mass of the LG from the orbit of the Milky Way and M31. We discuss a number of systematic effects. The first is caused by the presence of the Large Magellanic Cloud (LMC). The interaction of the LMC with the Milky Way induces a motion toward the LMC. This contribution to the measured velocity of approach of the Milky Way and M31 must be removed. The second is the cosmological constant whose effects must be incorporated on these length scales. The third is to allow for cosmic bias and scatter, pre-conditioned by the accretion history of the LG. Without taking these into account, the timing argument significantly overestimates the true mass. Adjusting for all these effects, we give the estimated mass of the LG for two treatments of M31's tangential velocity. The first is $M =3.4^{+1.4}_{−1.1} \times 10^{12}M_⊙$ (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 $M=3.1^{+1.3}_{−1.0}×10^{12}M_⊙$ (68% CL). By making an inventory of the total mass associated with the four most substantial LG members (the Milky Way, M31, M33, and the LMC), we estimate the known mass to be in the range $3.7^{+0.5}_{−0.5} \times 10^{12}M_⊙$

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