HALO7D: Disentangling the Milky Way Accretion History with Observations in 7 Dimensions

The Milky Way (MW) is shrouded in a faint metal-poor stellar halo. Its structure and kinematics provide a unique archaeological record of the MW's formation, past evolution, and accretion history. However, studies of the MW stellar halo are hindered by observational constraints. Prior to the era of Gaia, beyond D~10 kpc, our knowledge of the MW halo was limited to line of sight (LOS) velocities and rare tracer populations (blue horizontal branch and red giant branch stars). However, even in the era Gaia, we are still missing tangential velocity information for halo main sequence stars. We aim to address these limitations with the HALO7D survey. The HALO7D dataset consists of highly accurate HST-measured proper motions (PMs) and very deep (8-24 hour integrations) Keck DEIMOS spectroscopy of MW main sequence turn-off stars in the CANDELS fields. This project provides the first opportunity to measure 6D phase-space information plus chemical abundances (7 "Dimensions") for distant halo main sequence stars. I will present the first kinematic results from this survey. We use the HALO7D dataset to estimate the parameters of the halo velocity ellipsoid, and explore how the velocity distributions and anisotropy vary across survey fields and as a function of distance. This survey will vastly improve our understanding of the Milky Way structure, evolution and mass in a way that neither the HST proper motions nor Keck spectroscopy can do on their own.

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