Till Sawala, Marius Cautun, Carlos Frenk, John Helly, Jens Jasche, Adrian Jenkins, Peter Johansson, Guilhem Lavaux , Stuart McAlpine, Matthieu Schaller

                    Nature Astronomy, in press, 2022NatAs.tmp.273S (arXiv.2205.02860)

In the 1970s, the great Cambridge astronomer, the late Professor Donald Lynden-Bell, noted that the 11 bright satellites orbiting the Milky Way seem to be arranged in an implausibly thin plane piercing through our galaxy – the Milky Way’s “plane of satellites”. To add to the mystery, it was later argued that these galaxies are circling the Galaxy in a coherent, long-lived disk. These observations became known as the “plane of satellites problem” of the standard cosmological model, LCDM, wherein the Galaxy is surrounded by a roughly spherical, dispersion-supported dark matter halo.

Positions and orbits of the 11 classical satellite galaxies of the Milky Way seen “edge-on”, integrated for 1 billion years into the past and future. The right panel is a zoom-in of the left panel. The black dot marks the centre of the Milky Way, arrows mark the observed positions and the directions of travel of the satellites. While they currently line up in a plane (indicated by the grey horizontal line), that plane quickly dissolves as the satellites move along their orbits.

We have shown that the reported exceptional anisotropy of the Milky Way satellite system is strongly contingent on its lopsided radial distribution, combined with the close but fleeting conjunction of the two most distant satellites, Leo I and Leo II. Using Gaia proper motions, we show the plane of satellites to be transient rather than rotationally supported.

One of the new high-resolution simulations of the dark matter enveloping the Milky Way and its neighbour, the Andromeda galaxy. The new study shows that earlier, failed attempts to find counterparts of the plane of satellites which surrounds the Milky Way in dark matter simulations was due to a lack of resolution.

We carried out 202 high-resolution cosmological zoom-in constrained simulations on COSMA-8, based on initial conditions designed to reproduce Local Group analogues within the observed large-scale structure. We show that the failure of previous simulations to find thin, seemingly rotationally-supported satellite planes is entirely due to their limited resolution. We address this shortcoming by using the GALFORM semi-analytic galaxy formation model to identify “orphan” satellites whose dark matter halos have been artificially disrupted. In this way, the radial distribution of the satellites in our simulations is consistent with the Milky Way data. Our simulations demonstrate that satellite alignments are short-lived, just as inferred for the Milky Way. Finally, the simulations reveal that planes of satellites as thin as that of the Milky Way and whose orbital poles have a similar degree of spatial coherence as in the Milky Way are not uncommon in LCDM. Rather, the failure to find them in previous simulations was due to resolution limitations in the very dense central regions of halos.

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