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We identify Local Group (LG) analogs in the IllustrisTNG cosmologicalsimulation, and use these to study two mass estimators for the LG: one based onthe timing argument (TA) and one based on the virial theorem (VT). Includingupdated measurements of the Milky Way-M31 tangential velocity and thecosmological constant, we show that the TA mass estimator slightlyoverestimates the true median LG-mass, though the ratio of the TA to the truemass is consistent at the approximate 90\% c.l. These are in broad agreementwith previous results using dark matter-only simulations. We show that the VTestimator better estimates the true LG-mass, though there is a larger scatterin the virial mass to true mass ratio relative to the corresponding ratio forthe TA. We attribute the broader scatter in the VT estimator to severalfactors, including the predominantly radial orbits for LG satellite galaxies,which differs from the VT assumption of isotropic orbits. With the systematicuncertainties we derive, the updated measurements of the LG mass at 90\% c.l.are $4.75_{-2.41}^{+2.22} \times 10^{12}$ M$_\odot$ from the TA and$2.0_{-1.5}^{+2.1} \times 10^{12}$ M$_\odot$ from the VT. We consider the LMC'seffect on the TA and VT LG mass estimates, and do not find exact LMC-MW-M31analogues in the Illustris simulations. However, in LG simulations withsatellite companions as massive as the LMC we find that the effect on the TAand VT estimators is small, though we need further studies on a larger sampleof LMC-MW-M31 systems to confirm these results.

Local Group timing argument and virial theorem mass estimators from cosmological simulations