The phase-space distribution of the M 81 satellite system

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. Comoving planes of satellites have been found around the Milky Way, the Andromeda galaxy, and the nearby Cen A galaxy, which seems to be at odds with the standard model of galaxy formation. Another nearby galaxy group, with a putative flattened distribution of dwarf galaxies, is the M 81 group. In this paper, we present a quantitative analysis of the distribution of the M 81 satellites using a Hough transform to detect linear structures. Using this method, we confirm a flattened distribution of the dwarf galaxies. Depending on the morphological type, we find a minor-to-major axis ratio of the satellite distribution of 0.5 (all types) or 0.3 (dSph), which is in line with previous results for the M 81 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. Furthermore, the satellite system seems to be lopsided. Employing line-of-sight velocities for a subsample of the dwarfs, we find no signal of corotation. Comparing the flattening and motion of the M 81 dwarf galaxy system with TNG50 of the IllustrisTNG suite we find good agreement between observations and simulations, but caution that i) velocity information of half of the satellite population is still missing, ii) current velocities mainly come from dwarf irregulars clustered around NGC 3077, which may indicate an infall of a dwarf galaxy group, and iii) some of the dwarfs in our sample may be tidal dwarf galaxies. From the missing velocities, we predict that the observed frequency within IllustrisTNG may still range between 2 to 29%. Any final conclusions about the agreement or disagreement with cosmological models needs to wait for a more complete picture of the dwarf galaxy system.