The structure of the halo of the Milky Way has recently been shown to be clearly divisible into two overlapping stellar components, the inner and the outer halo. The first structure dominates at Galactocentric distances R < 10-15 kpc, exhibits highly eccentric orbits, has a slightly pro-grade rotation, and comprises stars with a peak in their metallicity distribution function (MDF) around [Fe/H] = -1.6. The outer-halo component dominates at R > 15-20 kpc, exhibits a much more uniform distribution of orbital eccentricities, has a clear (and statistically significant) net retrograde rotation, and comprises stars with a peak in their MDF a factor of three lower than the inner halo ([Fe/H] = -2.2). Such properties indicate that one might associate two distinct modes of formation, and timescales, for the assembly of these two structures. The dataset upon which these claims are based has recently been increased by 50% with the addition of new stars from SDSS/SEGUE. We present the first results of a Maximum Likelihood technique that provides estimates for the structural parameters of the inner- and outer-halo components, including estimates of the fraction of stars within each population as a function of distance. Such numbers are critical for refining ongoing and future searches for the most metal-poor stars in the Galaxy.
|Journal||Proceedings of Science|
|Publication status||Published - 2008|