Dwarf satellite galaxies are thought to be the remnants of the population of primordial
structures that coalesced to form giant galaxies like the Milky Way1. An early analysis2 noted
that dwarf galaxies may not be isotropically distributed around our Galaxy, as several are
correlated with streams of HI emission, and possibly form co-planar groups3. These
suspicions are supported by recent analyses4–7, and it has been claimed7 that the apparently
planar distribution of satellites is not predicted within standard cosmology8, and cannot
simply represent a memory of past coherent accretion. However, other studies dispute
this conclusion9–11. Here we report the existence (99.998% significance) of a planar subgroup
of satellites in the Andromeda galaxy, comprising ≈ 50% of the population. The
structure is vast: at least ∼ 400 kpc in diameter, but also extremely thin, with a
perpendicular scatter < 14.1 kpc (99% confidence). Radial velocity measurements12–15 reveal
that the satellites in this structure have the same sense of rotation about their host. This
finding shows conclusively that substantial numbers of dwarf satellite galaxies share the same
dynamical orbital properties and direction of angular momentum, a new insight for our
understanding of the origin of these most dark matter dominated of galaxies. Intriguingly,
the plane we identify is approximately aligned with the pole of the Milky Way’s disk and is
co-planar with the Milky Way to Andromeda position vector. The existence of such extensive
coherent kinematic structures within the halos of massive galaxies is a fact that must be
explained within the framework of galaxy formation and cosmology.
arXiv
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