Spatial and kinematic alignments between central and satellite halos

Based on a cosmological N-body simulation, we analyze spatial and kinematic alignments of satellite halos within 6 times the virial radius of group-sized host halos (r(vir)). We measure three different types of spatial alignment: halo alignment between the orientation of the group central substructure (GCS) and the distribution of its satellites, radial alignment between the orientation of a satellite and the direction toward its GCS, and direct alignment between the orientation of the GCS and that of its satellites. Analogously, we use the directions of satellite velocities and probe three further types of alignment: the radial velocity alignment between the satellite velocity and the connecting line between the satellite and GCS, the halo velocity alignment between the orientation of the GCS and satellite velocities, and the autovelocity alignment between the satellite orientations and their velocities. We find that satellites are preferentially located along the major axis of the GCS within at least 6r(vir) ( the range probed here). Furthermore, satellites preferentially point toward the GCS. The most pronounced signal is detected on small scales, but a detectable signal extends out to similar to 6r(vir). The direct alignment signal is weaker; however, a systematic trend is visible at distances less than or similar to 2r(vir). All velocity alignments are highly significant on small scales. The halo velocity alignment is constant within 2r(vir) and declines rapidly beyond. The halo and the autovelocity alignments are maximal at small scales and disappear beyond 1r(vir) and 1.5r(vir), respectively. Our results suggest that the halo alignment reflects the filamentary large-scale structure that extends far beyond the virial radii of the groups. In contrast, the main contribution to the radial alignment arises from the adjustment of the satellite orientations in the group tidal field. The projected data reveal good agreement with recent results derived from large galaxy surveys.