The halo of the Milky Way provides unique elemental abundance and kinematic information on the first objects to form in the Universe, and this information can be used to tightly constrain models of galaxy formation and evolution. Although the halo was once considered a single component, evidence for its dichotomy has slowly emerged in recent years from inspection of small samples of halo objects. Here we show that the halo is indeed clearly divisible into two broadly overlapping structural components-an inner and an outer halo-that exhibit different spatial density profiles, stellar orbits and stellar metallicities (abundances of elements heavier than helium). The inner halo has a modest net prograde rotation, whereas the outer halo exhibits a net retrograde rotation and a peak metallicity one-third that of the inner halo. These properties indicate that the individual halo components probably formed in fundamentally different ways, through successive dissipational (inner) and dissipationless (outer) mergers and tidal disruption of proto-Galactic clumps.Astronomers have long sought to constrain models for the formation and evolution of the Milky Way (our Galaxy) on the basis of observations of the stellar and globular cluster populations that it contains. These populations are traditionally defined as samples of objects that exhibit common spatial distributions, kinematics and metallicities (the age of a population, when available, is also sometimes used). Metallicity is taken by astronomers to represent the abundances of elements heavier than helium, which are only created by nucleosynthesis in stars-either internally via nuclear burning in their cores or externally during explosive nucleosynthesis at the end of their lives. The earliest generations of stars have the lowest metallicities, because the gas from which they formed had not been enriched in heavy elements created by previous stars and distributed throughout the primordial interstellar medium by stellar winds and supernovae.Previous work has provided evidence that the halo of the Milky Way may not comprise a single population, primarily from analysis of the spatial profiles (or inferred spatial profiles) of halo objects [1][2][3][4] . A recent example of such an analysis is the observation of two different spatial density profiles for distinct classes of RR Lyrae variable stars in the halo 5 . In addition, tentative claims for a net retrograde motion of halo objects by previous authors supports the existence of a likely dual-component halo [6][7][8][9][10] . The central difficulty in establishing with confidence whether or not a dichotomy of the halo populations exists is that the past samples of tracer objects have been quite small, and usually suitable only for consideration of a limited number of the expected signatures of its presence.In the present work, we examine this question in detail using a large, homogeneously selected and analysed sample of over 20,000 stars, originally obtained as calibration data during the course of the Sloan Digital Sk...

We derive new constraints on the mass of the Milky Way's dark matter halo, based on a set of halo stars from SDSS as kinematic tracers. Our sample comprises 2401 rigorously selected Blue Horizontal-Branch (BHB) halo stars at |z| ≥ 4 kpc, and with distances from the Galactic center up to ∼ 60 kpc, with photometry and spectra drawn from SDSS DR-6. With distances accurate to ∼ 10%,

We describe the development and implementation of the SEGUE (Sloan Extension for Galactic Exploration and Understanding) Stellar Parameter Pipeline (SSPP). The SSPP derives, using multiple techniques, radial velocities and the fundamental stellar atmospheric parameters (effective temperature, surface gravity, and metallicity) for AFGK-type stars, based on medium-resolution spectroscopy and ugriz photometry obtained during the course of the original Sloan Digital Sky Survey (SDSS-I) and its Galactic extension (SDSS-II/SEGUE). The SSPP also provides spectral classification for a much wider range of stars, including stars with temperatures outside of the window where atmospheric parameters can be estimated with the current approaches. This is Paper I in a series of papers on the SSPP; it provides an overview of the SSPP, and initial tests of its performance using multiple data sets. Random and systematic errors are critically examined for the current version of the SSPP, which has been used for the sixth public data release of the SDSS (DR-6).

The high-and intermediate-velocity interstellar clouds (HVCs/IVCs) are tracers of energetic processes in and around the Milky Way. Clouds with near-solar metallicity about 1 kpc above the disk trace the circulation of material between disk and halo (the Galactic fountain). The Magellanic Stream consists of gas tidally extracted from the SMC, tracing the dark matter potential of the Milky Way. Several other HVCs have low metallicity and appear to trace the continuing accretion of infalling intergalactic gas. These assertions are supported by the metallicities (0

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