Bar-driven dispersal of Galactic substructure

TL;DR

This paper investigates how the Milky Way's rotating bar affects the dispersal of galactic substructures, showing that it significantly spreads their energy and angular momentum, and proposes new methods for identifying these dispersed structures.

Contribution

It quantifies the bar's impact on substructure dispersal using multiple models and suggests using Jacobi integral and chemistry for better detection of dispersed substructures.

Findings

Bar increases energy and angular momentum spread of substructures by 10-100 times.

Most affected orbits are low energy, prograde, eccentric, or low inclination.

Substructure dispersal aligns with the bar's pattern speed in energy-angular momentum space.

Abstract

Galactic archaeologists often assume that integrals of motion (IoMs) such as $L_z$ and $E$ are conserved, so substructure remains frozen in IoM space over many Gyr. However, this is not true in the Milky Way due in part to its rotating bar. In this study we quantify the effects of the bar on the dynamics of substructure. We employ three different theoretical models: an analytical toy model; a set of test particle simulations with steady and slowing bars; and a cosmological zoom-in simulation of a Milky Way-like galaxy. Each model predicts that the bar increases the angular momentum and energy spread of low-energy substructures by a factor of $\sim10-100$, so they cannot remain tightly clustered. We derive a criterion for determining when this effect is important. The most affected orbits are low energy ($E\lesssim E_\odot$, $r_\mathrm{apo}<40$ kpc), prograde, eccentric, or low inclination. This includes $\sim3/4$ of Galactic globular clusters and $\sim1/4$ of known stellar streams. We predict the presence of abundant bar-dispersed substructure. The structures remain much more tightly clustered in the space of metallicity and Jacobi integral $H_\mathrm{J}=E-\Omega_\mathrm{b}L_z$. We therefore propose using $H_\mathrm{J}$ and chemistry instead of traditional IoMs when searching for inner halo substructure. In $(L_z,E)$ space the dispersal of the structures is along a principal direction with gradient $\mathrm{d}E/\mathrm{d}L_z$ equal to the bar's pattern speed $\Omega_\mathrm{b}$. Bar-dispersed substructure should therefore allow the past evolution of $\Omega_\mathrm{b}$ to be constrained.