Stellar halo striations from assumptions of axisymmetry

TL;DR

This paper investigates how assuming axisymmetry in the galactic potential leads to artificial stellar halo features called striations, and suggests these features can help constrain the true shape of the halo.

Contribution

It demonstrates that axisymmetric assumptions produce artificial halo striations in simulations, which depend on the potential's shape, offering a new way to test halo symmetry.

Findings

Striations form in (L_z, E) space under axisymmetric assumptions.

Including a quadrupole component reduces the prominence of these striations.

Absence of striations in data suggests the halo may not be axisymmetric.

Abstract

Motivated by the LMC's impact on the integral of motion space of the stellar halo, we run an $N$-body merger simulation to produce a population of halo-like stars. We subsequently move to a test particle simulation, in which the LMC perturbs this debris. When an axisymmetric potential is assumed for the final snapshot of the $N$-body merger remnant, a series of vertical striations in $(L_z, E)$ space form as the LMC approaches its pericentre. These result from the formation of overdensities in angular momentum owing to a relationship between the precession rate of near radial orbits and the torquing of these orbits by the LMC. This effect is heavily dependent on the shape of the inner potential. If a quadrupole component of the potential is included these striations become significantly less apparent due to the difference in precession rate between the two potentials. The absence of these features in data, and the dramatic change in orbital plane precession rate, discourages the use of an axisymmetric potential for highly eccentric orbits accreted from a massive GSE-like merger. Given the link between appearance of these striations and the shape of the potential, this effect may provide a new method of constraining the axisymmetry of the halo.