Abstract

Halo stars may have formed in situ, been flung out from further in, or accreted from neighbouring systems. The particular combination of processes is embedded in the phase-space and metallicity distributions of the stars, and therefore they serve as fossils of the halo’s formation history. I will present models of the haloes of nearby massive elliptical galaxies and our own Galaxy, using a range of methods to constrain the dark matter mass, the structure of stellar orbits, and examine age and metallicity gradients. We find a strong correlation between the dark matter mass and the density of the local environment, and evidence for generally radial orbits, both consistent with an accretion scenario for halo formation. Higher ages and metallicities are clearly more concentrated in action space, where actions are constants of motion such as angular momentum. This favours a scenario where more massive systems with older and metal-richer stars penetrate the gravitational potential of the central system more deeply. However the gradients manifest in real space only as a negligible metallicity gradient (_{-0.001 dex/kpc) and a weak age gradient (}-0.02 Gyr/kpc).