Abstract

Recent work uncovered features in the phase space of the Milky Way’s stellar halo which may be attributed to the last major merger. When stellar material from a satellite is accreted onto its host, it phase mixes and appears finely substructured in phase space. For a high-eccentricity merger, this substructure most clearly manifests as numerous wrapping chevrons in (𝑣𝑟 , 𝑟) space, corresponding to stripes in (𝐸, 𝜃𝑟 ) space. We introduce the idea of using this substructure as an alternative subhalo detector to cold stellar streams. We simulate an N-body merger akin to the Gaia Sausage / Enceladus and assess the impact of subhaloes on these chevrons. To quantify the impact of perturbers, we utilise the simpler appearance of the chevrons in (𝐸, 𝜃𝑟 ) space. We show that a single flyby of a massive ( _{1010 M⊙) subhalo with pericentre comparable to, or within, the chevron’s apocentre smooths out the substructure, whereas a single flyby of a low mass (} 108 M⊙) has negligible effect. The effects of known massive perturbers (e.g. Sagittarius) could be detectable and offer constraints on their initial mass.