The SIBELIUS Project: E Pluribus Unum

Abstract

We introduce "Simulations Beyond The Local Universe" (SIBELIUS) that connect the Local Group to its cosmic environment. We show that introducing hierarchical small-scale perturbations to a density field constrained on large scales by observations provides an efficient way to explore the sample space of Local Group analogues. From more than 60 000 simulations, we identify a hierarchy of Local Group characteristics emanating from different scales: the total mass, orientation, orbital energy and the angular momentum are largely determined by modes above $\lambda$ = 1.6 comoving Mpc (cMpc) in the primordial density field. Smaller scale variations are mostly manifest as perturbations to the MW-M31 orbit, and we find that the observables commonly used to describe the Local Group -- the MW-M31 separation and radial velocity -- are transient and depend on specifying scales down to 0.2 cMpc in the primordial density field. We further find that the presence of M33/LMC analogues significantly affects the MW-M31 orbit and its sensitivity to small-scale perturbations. We construct initial conditions that lead to the formation of a Local Group whose primary observables precisely match the current observations.

14 pages, 15 figures. Submitted to MNRAS, comments welcome!

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• Implications of the Milky Way travel velocity for dynamical mass estimates of the Local Group
• Constraints on dark matter annihilation and decay from the large-scale structure of the nearby universe
• Matching the mass function of Milky Way satellites in competing dark matter models
• The effect of local universe constraints on halo abundance and clustering
• The north-south asymmetry of the ALFALFA HI velocity width function