SIBELIUS

Sibelius Overview

In the “Simulations Beyond The Local Universe” (SIBELIUS) project the aim is to connect the Local Group of galaxies to its true cosmic environment. This is achieved by performing constrained local universe simulations for which the phase information has been obtained using the BORG algorithm (Bayesian Origin Reconstruction from Galaxies).

The SIBELIUS project is funded by the Academy of Finland via grants no. 354905, 335607, 314238 and 311049.

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The SIBELIUS project thus connects the Local Group to its environment using a large set of constrained cosmological zoom-in simulations. Read more: The SIBELIUS Project: E Pluribus Unum.

Duplicates of an LG analogue from the 3.2-cMpc variations. The top left-hand panel shows the projected DM density around the Local Group for variation i = 622, highlighting in red and yellow particles within 250 kpc of the centres of the MW and M31 analogue halos. The other three panels show the dark matter density in three other realizations of the 3.2-cMpc set, at the same z = 0 coordinates, and with the same particles highlighted. All four LG analogues share more than 50 per cent of particle IDs, i.e. they have formed from the same Lagrangian volume, and also have similar mass and orientation. Adapted from Sawala et al. 2022, MNRAS, 509, 1432.

In addition we have also run a simulation named SIBELIUS-DARK, which is the largest and most comprehensive ‘constrained realisation’ simulation to date. It covers a volume up to a distance 600 million lightyears from Earth and is represented by over 130 billion simulated ‘dark matter particles’, requiring many thousands of computers working in tandem over several weeks and producing over 1 Petabyte of data. It was performed on the DiRAC COSmology MAchine (COSMA) operated by the Institute for Computational Cosmology of Durham University. Read more on the Sibelius Dark website and in the research paper SIBELIUS-DARK: a galaxy catalogue of the local volume from a constrained realization simulation.

The dark matter distribution of the entire sibelius-dark volume (dMW ≤ 200 Mpc), viewed in six spherical shells centred on the Milky Way (blue/green). Each slice is presented as an all-sky-map in a Mollweide projection using the Galactic coordinate system. Overplotted as red points are the galaxies from the 2M++ galaxy sample (Lavaux & Hudson 2011), demonstrating just how well the non-linear structure of the local volume has been encapsulated in the borg reconstruction. The locations of twelve famous clusters/concentrations are also highlighted. Adapted from McAlpine et al. 2022, MNRAS, 512, 5823.

Selected Research Papers

The uncertain future of the Milky Way-Andromeda collision

It is commonly believed that our own Milky Way is on a collision course with the neighbouring Andromeda galaxy. As a result of their merger, predicted in around 5 billion years, the two large spiral galaxies that define the present Local Group would form a new elliptical galaxy. In this paper we consider the latest and most accurate observations by the Gaia and Hubble space telescopes, along with recent consensus mass estimates, to derive possible future scenarios and identify the main sources of uncertainty in the evolution of the Local Group over the next 10 billion years. We found that the next most massive Local Group member galaxies—namely, M33 and the Large Magellanic Cloud—distinctly and radically affect the Milky Way-Andromeda orbit. Although including M33 increases the merger probability, the orbit of the Large Magellanic Cloud runs perpendicular to the Milky Way-Andromeda orbit and makes their merger less probable. In the full system, we found that uncertainties in the present positions, motions and masses of all galaxies leave room for drastically different outcomes and a probability of close to 50% that there will be no Milky Way-Andromeda merger during the next 10 billion years. Based on the best available data, the fate of our Galaxy is still completely open

On each panel, we show 100 realizations of our fiducial Monte Carlo model and also state the probability for a MW-M31 merger within 10 Gyr. In the top row, we show the MW-M31 two-body orbits (top left) and MW-M31-M33-LMC four-body orbits (top right). In the bottom row, we show the MW-M31-M33 (bottom left) and MW-M31-LMC (bottom right) three-body orbits. White markers denote MW-M31 mergers. Percentages indicate the fraction of orbits that merge within 10 Gyr (fM). Only slightly more than half of the four-body orbits lead to a merger within 10 Gyr. The inclusion of M33 increases the likelihood of a merger, whereas the inclusion of the LMC decreases it. White lines show the individual orbits using the most probable values of every variable, either assuming the Gaia DR3 proper motions29 of the fiducial model (solid) or HST + Gaia DR2 proper motions13 (dashed). ML, fiducial model. Adapted from Sawala et al., 2025, Nature Astronomy, in press.

The Milky Way plane of satellites

The Milky Way is surrounded by 11 classical satellite galaxies in a remarkable configuration: a thin plane that is possibly rotationally supported. Such a structure is thought to be highly unlikely to arise in the standard (ΛCDM) cosmological model (Λ cold dark matter model, where Λ is the cosmological constant). This is the so-called `plane of satellites problem’, which challenges not only the ΛCDM model but the entire concept of dark matter. In a recent Nature Astronomy paper we show that the reportedly exceptional anisotropy of the Milky Way satellites can be explained, in large part, by their lopsided radial distribution combined with the temporary conjunction of the two most distant satellites, Leo I and Leo II. Using Gaia proper motions, we show that the orbital pole alignment is much more common than previously reported, and reveal the plane of satellites to be transient rather than rotationally supported. Comparing with new simulations, where such short-lived planes are common, we find the Milky Way satellites to be compatible with standard model expectations.

Arrowheads show maximum likelihood positions, projected face-on (top panels) and edge-on (bottom panels) according to the eigenvectors of the full inertia tensor. Lines show orbits integrated for 1 Gyr into the past and future. Faint lines show 200 Monte Carlo samples of the observations and bold lines show the maximum likelihood orbit. The left panels show the orbits of the 4 satellites with Galactocentric distances beyond 100 kpc and the right panels show the remaining 7 orbits in an inset of ±100 kpc around the Galactic Centre. With the Gaia EDR3 measurements, the proper motions are very well constrained, with the exception of the LMC and SMC. It can also be seen that the MW satellites are highly concentrated, with 7 out of 11 within 100 kpc and only 2, Leo I and Leo II, at r > 200 kpc. Several galaxies, including Leo I and II, are presently crossing the ‘plane’ (indicated by the grey horizontal lines in the bottom panels), which soon disperses as a result. Adapted from Sawala et al., 2023, Nature Astronomy, 7, 481.

The Formation of the Local Group of Galaxies

We study the Local Group (LG) of galaxies using numerical simulations, as it provides us with a unique window into the Universe. In the Local Group we can observe the faintest galaxies, which enable us to study the nature of dark matter. In particular, the abundance, anisotropy and inner structure of LG galaxies have all been considered challenges to the prevailing Lambda cold dark matter paradigm. While recent advanced in cosmological hydrodynamical simulations indicate that baryonic effects may resolve some of these issues, other questions remain, such as the peculiar orbits and alignments of dwarf galaxies within the Local Group.