Spatial distribution of DM particles in MW 550149, focusing on the Traceable merger highlighted in the middle panel of Figure 4. This merger’s DM particles are shown 2 Gyr before infall (left), at infall (middle), and 2 Gyr after infall (right). All panels display the same pool of 58,541 DM particles that SUBFIND identifies as bound to the merger 2 Gyr before infall, subsampled such that they are more visible in this image. We apply the DM tracking algorithm to divide the particles into three categories: tagged to this merger (blue filled circles), untagged (gray unfilled circles), and tagged to another merger (orange triangles). The host MW is indicated by a point (purple) with its virial radius shown as a circle (dashed purple). Of the selected particles, 96% are tagged to this merger, 4% are not tagged to any merger, and <0.1% are tagged to another merger. This example demonstrates that the tracking procedure robustly associates the majority of DM particles with their correct merger, with very few left untagged or mistagged. Note that this figure assesses the completeness of the algorithm rather than the purity, as particles that are not bound to this merger 2 Gyr before infall are not shown.