Galactic halo stars in phase space: A hint of satellite accretion?

The present-day chemical and dynamical properties of the Milky Way bear the imprint of the Galaxy's formation and evolutionary history. One of the most enduring and critical debates surrounding Galactic evolution is that regarding the competition between "satellite accretion" and "monolithic collapse"; the apparent strong correlation between orbital eccentricity and metallicity of halo stars was originally used as supporting evidence for the latter. While modern-day unbiased samples no longer support the claims for a significant correlation, recent evidence has been presented by Chiba & Beers for the existence of a minor population of high-eccentricity metal-deficient halo stars. It has been suggested that these stars represent the signature of a rapid (if minor) collapse phase in the Galaxy's history. Employing velocity and integrals of motion phase-space projections of these stars, coupled with a series of N-body/smoothed particle hydrodynamic chemodynamical simulations, we suggest that an alternative mechanism for creating such stars may be the recent accretion of a polar orbit dwarf galaxy.