Creation and destruction of halo substructure by the Galactic bar

Abstract: The bar at the centre of the Milky Way has an enormous influence on the Galaxy. In this talk I discuss some of its effects on the stellar halo. Firstly, I show how the bar produces dynamical streams of resonantly trapped stars on highly eccentric orbits, analogous to moving groups in the disc such as the Hercules stream. These features are observable as asymmetries in the velocity space of halo stars. By fitting resonant orbits to these streams across a range of radii, the bar’s pattern speed and the Galactic potential at radii of r < 20 kpc are simultaneously constrained. Secondly, I demonstrate how the bar can disperse halo substructure in the space of integrals of motion, such as (L_z, E). By running test particle simulations of stars being tidally stripped from globular clusters, I show that stars from a given progenitor do not remain clustered in energy or angular momentum. Instead, they spread out in (L_z, E) space along lines of gradient equal to the bar’s pattern speed, due to conservation of the Jacobi integral. The effect is most pronounced at energies below that of the Sun and on prograde orbits. This has serious implications for the use of dynamical quantities in identifying unbound substructure in the inner halo. To perform Galactic archaeology at low energies, a combination of chemistry and the Jacobi integral should be used instead of the traditional integrals of motion.

This seminar is organised by the Ursa Minor research group and is open to all.