  The spatial distribution of dwarf galaxies around their host galaxies is a
critical test for the standard model of cosmology because it probes the
dynamics of dark matter halos and is independent of the internal baryonic
processes of galaxies. Co-moving planes-of-satellites have been found around
the Milky Way, the Andromeda galaxy, and the nearby CenA galaxy, which seem to
be at odds with the standard model of galaxy formation. Another nearby galaxy
group, with a putative flattened distribution of dwarfs, is the M81 group. We
present a quantitative analysis of the distribution of the M81 satellites using
a Hough transform to detect linear structures. We confirm a flattened
distribution of dwarf galaxies. Depending on the morphological type, we find a
minor-to-major axis ratio of the satellite distribution to be 0.5 (all types)
or 0.3 (dSph), which is in line with previous results for the M81 group.
Comparing the orientation of this flattened structure in 3D with the
surrounding large-scale matter distribution, we find a strong alignment with
the local sheet and the planes-of-satellites around the Andromeda galaxy and
Cen A. Employing velocities for a sub-sample of the dwarfs, we find no signal
of co-rotation. Comparing the flattening and motion of the M81 dwarf galaxy
system with IllustrisTNG50 we find good agreement between observations and
simulations, but caution that i) velocity information of half of the satellites
is missing, ii) velocities are coming mainly from dwarf irregulars clustered
around NGC3077, which may hint towards an infall of a dwarf galaxy group and
iii) some of the dwarfs may actually be tidal dwarf galaxies. From the missing
velocities, we predict that the observed frequency within TNG may range between
2 to 29 per cent. Any conclusions about the agreement/disagreement with
cosmological models needs to wait for a more complete picture of the dwarf
galaxy system.
