The SIBELIUS Project: E Pluribus Unum

TL;DR

The SIBELIUS project uses large-scale constrained simulations to explore the formation and characteristics of Local Group analogues, revealing how different scales influence key properties and the importance of small-scale perturbations.

Contribution

This work introduces a hierarchical simulation approach that connects large-scale cosmic structures to local group features, providing new insights into the origins of their observed properties.

Findings

Mass, orientation, and orbital energy are mainly determined by large-scale modes.

Small-scale variations influence the MW-M31 orbit and observables.

Constructed initial conditions produce Local Groups matching current observations.

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.