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

TL;DR

This study uses cosmological simulations to evaluate two mass estimation methods for the Local Group, finding the virial theorem estimator more accurate but with larger scatter, and providing updated mass estimates with uncertainties.

Contribution

It introduces a comparison of timing argument and virial theorem mass estimators for the Local Group using IllustrisTNG simulations, including effects of the LMC and updated measurements.

Findings

TA slightly overestimates true mass, consistent at 90% c.l.

VT estimator better estimates true mass but with larger scatter.

Updated LG mass estimates: ~4.75e12 and ~2.0e12 solar masses.

Abstract

We identify Local Group (LG) analogs in the IllustrisTNG cosmological simulation, and use these to study two mass estimators for the LG: one based on the timing argument (TA) and one based on the virial theorem (VT). Including updated measurements of the Milky Way-M31 tangential velocity and the cosmological constant, we show that the TA mass estimator slightly overestimates the true median LG-mass, though the ratio of the TA to the true mass is consistent at the approximate 90\% c.l. These are in broad agreement with previous results using dark matter-only simulations. We show that the VT estimator better estimates the true LG-mass, though there is a larger scatter in the virial mass to true mass ratio relative to the corresponding ratio for the TA. We attribute the broader scatter in the VT estimator to several factors, including the predominantly radial orbits for LG satellite galaxies, which differs from the VT assumption of isotropic orbits. With the systematic uncertainties 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's effect on the TA and VT LG mass estimates, and do not find exact LMC-MW-M31 analogues in the Illustris simulations. However, in LG simulations with satellite companions as massive as the LMC we find that the effect on the TA and VT estimators is small, though we need further studies on a larger sample of LMC-MW-M31 systems to confirm these results.