The outer region of the Milky Way, beyond the disk systems, has long been
thought of as a single entity -- comprising old stars and globular
clusters that represent the earliest populations of objects to have
formed in our Galaxy.
Previous studies of the halo have been limited by the small numbers of stars that could be confidently identified as members, and also by the lack of observed spectroscopy from which radial velocities and estimates of atmospheric parameters (such as temperature, surface gravity, and metallicity) could be obtained. Based on a very large set of new spectroscopy for stars in the halo of the Galaxy, obtained with the Sloan Digital Sky Survey, we show that the halo is clearly divisible into two broadly overlapping structural components. These are the inner halo, which is dominated by stars on highly eccentric orbits and exhibits a peak metallicity [Fe/H] = -1.6, as well as a somewhat flattened density distribution with a net prograde rotation, and the outer halo, which is dominated by stars on lower eccentricity orbits, exhibits a peak metallicity [Fe/H] = -2.2, and a spherical density distribution with a very high retrograde rotation. These results confirm expectations, based on the hierarchical Cold Dark Matter paradigm, that the outer halo of our Galaxy is likely to have been accreted from smaller sub-systems, perhaps similar to recently discovered low-luminosity dwarf spheroidal galaxies.
I will also comment on recent determinations of photometric metallicity estimates for SDSS stars, which has enabled the construction of metallicity maps of the disk and halo populations of the Galaxy (involving millions of stars), and will likely become a fundamental technique in the LSST era.