The Local Group’s mass: probably no more than the sum of its parts

The total mass of the Local Group (LG) and masses its primary constituents, Milky Way M31, are important anchors for several cosmological questions. In recent years, independent measurements have consistently yielded halo close to $10^{12} \mathrm{M_\odot}$ MW, $1-2 \times 10^{12} while estimates derived from pair's kinematics via `timing argument' a combined around $5 \mathrm{M_\odot}$. Here, we analyse extremely large Uchuu simulation constrain two most massive members. First, demonstrate importance selecting LG analogues whose dominated by mutual interactions similar extent as LG. Adopting observed separation radial velocity, obtain weighted posterior $75_{-40}^{+65}$ kms$^{-1}$ uncertain transverse velocity. Via Gaussian process regression, infer $3.2^{+1.2}_{-0.9} \mathrm{M_\odot}$, significantly below timing argument prediction. Importantly, show that remaining uncertainty is not rooted in analysis or observational errors, but irreducible scatter kinematics-mass relation. We further find less $0.9_{-0.3}^{+0.6} more $2.3_{-0.9}^{+1.0} consistent with MW respectively. Incorporating an additional prior allows us all determine very likely be lower object two.