Tidal Effects on the Spatial Structure of the Local Group

The spatial distribution of galaxies in the Local Group (LG) is the footprint of its formation mechanism and the gravitational interactions among its members and the external massive galaxies or galaxy groups. Recently, Pasetto & Chiosi (2007), using a 3D-geometrical description of the spatial distribution of all the members of the LG (not only the satellites of the MW and M31) based on present-day data on positions and distances, found that all galaxies (MW, M31, their satellites, and even the most distant objects) are confined within a slab of about 200 kpc thickness. Examining how external galaxies or groups would gravitationally affect (and eventually alter) the planar structure (and its temporal evolution) of the LG, they found that the external force field acts parallel to the plane determined by geometry and studied this with the Least Action Principle. In this paper, we have thoroughly investigated the role played by the tidal forces exerted by external galaxies or galaxy groups on the LG galaxies (the most distant dwarfs in particular) in shaping their large scale distribution. The idea based on the well known effect of tidal interactions, according to which a system of mass-points can undergo not only tidal stripping but also tidal compression and thus become flatter. Excluding the dwarf galaxies tightly bound to the MW and M31, the same tidal forces can account for the planar distribution of the remaining dwarf galaxies. We analytically recover the results of Pasetto & Chiosi (2007) and prove that a planar distribution of the LG dwarf galaxies is compatible with the external force field. We also highlight the physical cause of this result.