Armchair cartography - A map of the galactic halo based on observations of local, metal-poor stars

Abstract

A method is developed by which one can reconstruct a global model of that part of the galactic stellar halo which consists of stars on orbits taking them near the Sun. The model is constructed from observed positions and three-dimensional velocities of local, metal-poor halo stars in terms of a discrete sum of orbits. The method is applied to a sample of 118 local stars, selected without kinematic bias, with [Fe/H] ≤ - 1.5 and 〈[Fe/H]〉 ≃- 2.0. A Stäckel model of the galactic potential is developed in the paper, much facilitating the implementation of the above method. The density distribution of the reconstructed halo model is found to be well fitted by a power law ρ(R)∝R-3.3±0.2 in the intermediate halo (8 <20 kpc), indicating that more than 90 per cent of all stars in the intermediate halo are on orbits taking them near the Sun. The model is therefore a nearly complete model of the galactic stellar halo in this region. The intermediate halo is found to consist of two components: a main, nearly spherical component with axial ratio q = 0.85 ± 0.12 comprising the large majority of the mass in the halo and a highly flattened component, contributing about 40 per cent of the density at the Sun. The model predicts a typical value of the line-of-sight velocity dispersion towards the galactic poles of 120-130 km s-1 out to 10 kpc distance for very metal-poor stars (〈[Fe/H]〉 ≃ - 2.0). This is somewhat more than the 100-110 km s-1 found for nonlocal RR Lyrae and blue horizontal branch stars from observations in situ and significantly more than the 74 ± 10 km s-1 similarly found for K-giants at the SGP with 〈[Fe/H]〉 ≃ - 1.3. This result would indicate that the halo subsystems become 'hotter' (and presumably somewhat rounder) with decreasing metal content, implying a radial metallicity gradient in the halo and suggesting a dissipative formation scenario for the halo. As none of the above stellar subsystems is characterized by dynamically important rotational mean motions, the halo may have formed through an isotropic, dissipative collapse.