The shape-velocity alignment of satellites forged by tidal locking and dynamical friction

TL;DR

This study uses the TNG50 simulation to analyze how satellite galaxies align their shapes with their motion and position relative to their host galaxy, revealing stronger radial than orbital alignments influenced by mass and dynamical effects.

Contribution

It provides new insights into the shape and motion alignment of satellites, highlighting the roles of tidal locking, dynamical friction, and projection effects in observed alignments.

Findings

Radial alignment is stronger than orbital alignment.

Inward- and outward-moving satellites show contrasting orbital behaviors.

Observed alignment in MW satellites is due to projection effects.

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 MW satellites is due to projection effects, as the major axes of satellites lie within their orbital planes, approximately coplanar with the observer.