The H3 Spectroscopic Survey and the Origin of the Galaxy

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A main component of our Milky Way galaxy is its stellar halo, a spherical distribution of stars extending 10 times further than the stellar disk. Galaxy formation theories predict that our Milky Way's stellar halo was formed from "shredding" of smaller galaxies. However, this theory has not yet been confirmed observationally. In this project the investigators will measure the basic stellar properties (velocities, distances, and "chemistry") of 300,000 stars in the stellar halo. With these data they will provide a full inventory of the building blocks that formed our stellar halo. Stellar ages, which are measured as part of this project, will be used to deduce the timeline of star formation and galaxy merger events. Finally, the investigators will use these observations to predict the effect of the Milky Way's largest satellite companion, the Large Magellanic Cloud, on the stellar halo. Such observations may provide unique insight into the nature of dark matter and gravity. Educationally, this project will support a graduate student who will be trained on the analysis of stellar spectra and the application of dynamical models to the Galaxy. The project will also support the training of summer undergraduates from under-represented backgrounds as part of the Banneker Institute. The Banneker Institute has a summer program aimed at developing students’ research skills and social justice education.The “Hectochelle in the Halo at High Resolution” (H3) Survey will deliver high resolution spectra of stars in high Galactic latitude fields in the Northern hemisphere to measure radial velocities, spectrophotometric distances, [Fe/H], and [alpha/Fe] relative abundances. Determining these parameters will be used to explain the origin of structures in phase plus chemistry space. These observations will enable detailed constraints on the assembly history of the Galaxy, measurements of the mass profile of the outer Galaxy, the origin of the in-situ stellar halo, and will be sensitive to a variety of interesting, rare populations such as chemically peculiar stars and high velocity stars.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.