The preferred direction of infalling satellite galaxies in the Local Group

Using a high-resolution dark matter (DM) simulation of the Local Group, conducted within the framework of the Constrained Local UniversE Simulation (CLUES) project, we investigate the nature of how satellites of the Milky Way (MW) and M31 are accreted. Satellites of these two galaxies are accreted anisotropically on to the main haloes, entering the virial radius of their hosts, from specific 'spots' with respect to the large-scale structure. Furthermore, the material which is tidally stripped from these accreted satellites is also, at z = 0, distributed anisotropically and is characterized by an ellipsoidal subvolume embedded in the halo. The angular pattern created by the locus of satellite infall points and the projected z = 0 stripped dark matter is investigated within a coordinate system determined by the location of the Local Group companion and the simulated Virgo cluster across concentric shells ranging from 0.1 to 5 r(vir). Remarkably, the principal axis of the ellipsoidal subvolume shows a coherent alignment extending from well within the halo to a few rvir. A spherical harmonics transform applied to the angular distributions confirms the visual impression: namely, the angular distributions of both the satellites entry points and stripped DM for both haloes are dominated by the l = 2 quadrupole term, whose major principal axis is approximately aligned across the shells considered. It follows that the outer (r > 0.5r(vir)) structure of the main haloes of the Local Group composed of stripped material is closely related to the cosmic web, within which it is embedded. Given the very plausible hypothesis that an important fraction of the stellar halo of the MW has been accreted from satellite galaxies, the present results can be directly applied to the stellar halo of the MW and M31. We predict that the remnants of tidally stripped satellites should be embedded in streams of material composed of dark matter and stars. The present results can therefore shed light on the existence of satellites embedded within larger streams of matter, such as the Segue 2 satellite.