Local Group dynamical history in ΛCDM shows systematic velocity disagreements

A simulation study of 32 Local Group galaxies within ΛCDM estimates the Local Group mass as 4.33 (+0.37/−0.32) × 10^12 solar masses, with the Milky Way containing about 14% ± 7% of that mass, and reports that observed radial velocities exceed model predictions by a typical ~45 km s−1.

Researchers modeled the motions of 32 galaxies in the Local Group to constrain its mass and dynamical history. They evolved many simulated particles from a pure Hubble flow at redshift 9 and matched trajectories to present-day galaxy positions. The simulations are axisymmetric and include gravity from Centaurus A, and the analysis approximately accounts for IC 342, M81, the Great Attractor and the Large Magellanic Cloud. Predicted final velocities were compared with observed distances and radial velocity measurements. Key findings: - The total Local Group mass is estimated as 4.33 (+0.37/−0.32) × 10^12 solar masses, with the Milky Way contributing about 14% ± 7% of that mass. - Observed radial velocities systematically exceed the best-fitting ΛCDM model predictions, with a typical disagreement of 45.1 (+7.0/−5.7) km s−1 and a maximum discrepancy of 110 ± 13 km s−1 for the galaxy DDO 99. - The study reports that no plausible set of initial conditions in their ΛCDM simulations produces a good match to the sample's radial velocities. - Interactions between Local Group dwarf galaxies are judged unlikely to fully explain the velocity differences. - One possible explanation noted is a past close flyby between the Milky Way and Andromeda, a scenario that appears in some modified gravity theories but not in ΛCDM. - The authors also mention that gravitational slingshot encounters with fast-moving massive galaxies could plausibly accelerate some non-satellite Local Group galaxies. Summary: The results show a measurable mismatch between observed galaxy radial velocities and predictions from these ΛCDM-based models while providing an estimated total mass for the Local Group and the Milky Way fraction. This leaves open questions about the Group's past interactions and the origin of the excess velocities. Undetermined at this time.