Halo substructure, Dark Matter, extreme stars
Figure 1: Gaia-ESO science at high Galactic latitudes and in the outer Galaxy focuses on galaxy formation and assembly. This figure shows the “field of streams” image of the stellar distribution in the SDSS DR7 survey area (Belokurov et al, 2006, ApJ, 642, L137). Here turnoff stars are shown, colour coded by distance, with red being distant, blue closer. The wealth of halo structure, dominated by the complex tidal tails of the Sgr dSph galaxy, is evident.The Gaia-ESO survey will extend this photometric map to a multi-dimensional kinematics-abundance map.
Figure 2: Discovery data for the Sgr dSph galaxy (Ibata, Gilmore & Irwin, 1994, Nature, 370, 194). This, the only galaxy yet discovered in phase space, illustrates the complexity in inner Galaxy lines of sight, and the potential discovery space.
Recent surveys have revealed that the halos of both our own and other Local Group galaxies are rich in substructures (Belokurov et al., ApJ, 642, L137, 2006; Freeman & Bland-Hawthorn, 2002, ARA&A, 40, 487; Beers & Christlieb, 2005, ARA&A, 43, 531) (Figures 1 and 2). These not only trace the Galaxy’s past, but have enormous potential as probes of its gravitational field and hence as tracers of the still very uncertain distribution of dark matter (Helmi et al., 2004, ApJ, 610, L97). High precision radial velocities for many stars at latitudes |b| > 30° will lead to the discovery of more substructures. Their abundance patterns will indicate clearly whether a given structure represents a disrupted object and of which type, or has formed dynamically by resonant orbit-trapping. The kinematics of streams will place tight constraints on the distribution of dark matter. The local dark matter mass distribution will be substantially better determined than the current result (Kuijken & Gilmore, 1989, MNRAS, 239, 605). Furthermore, this large-number survey, with the metallicity-sensitive Calcium Triplet lines observed for every field star, will allow us to identify useful samples of rare kinematics or abundances for later follow-up (Tafelmeyer et al., 2011, A&A, 524, 58).
