The Shape-velocity Alignment of Satellites Forged by Tidal Locking and Dynamical Friction

Abstract Utilizing the TNG50 simulation, we study two types of alignments for satellites/subhalos: (1) the alignment of their major axes with the galactocentric radial directions (radial alignment), and (2) with the motion directions (orbital alignment). We find that radial alignment is substantially stronger than orbital alignment, with both signals being consistently stronger for subhalos than for satellites. Interestingly, inward- and outward-moving satellites/subhalos show contrasting orbital alignment behaviors, which can be understood in terms of their radial alignment, orbit decay due to dynamical friction, and the effect of tidal stripping. The orbital alignment is stronger in more-massive halos. In the end, we explore the orbital alignment measured by a mock observer, and find that the observed alignment for Milky Way satellites is due to projection effects, as the major axes of satellites lie within their orbital planes, approximately coplanar with the observer.