Decoding the Milky Way galaxy

Thursday, December 12, 2019 - 4:00pm
Phillips Auditorium
Harvard-Smithsonian CfA

Stars orbiting in the Milky Way halo comprise only 1% of our Galaxy's stellar mass. However, these stars are among the oldest in the Galaxy, and thanks to their long relaxation times, they preserve a historical record of the Milky Way forming. To access this historical memory of the halo, we need to measure the precise 3D positions, 3D velocities, and chemical abundances of large numbers of stars. Thanks to the Gaia mission, 5D positions are now available for almost 2 billion stars. In this talk, I will present first results from spectroscopic surveys that use Gaia data to preferentially target halo stars, and have so far mapped the full 6D phase-space for >70,000 stars. I will first discuss how a blind halo survey can be used to discover even the oldest, completely dissolved, progenitors of the Milky Way because they remain distinct in the space of conserved orbital quantities. The abundance and chemical properties of the Milky Way progenitors will provide a unique window into the early universe. Then, I will show how similar observations targeted at a known, cold stellar stream, suggest it recently had a close encounter with a massive and dense perturber, and also constrain the perturber's orbit and present-day location. Known baryonic objects are unlikely perturbers based on their orbital properties, but observations permit a low-mass dark-matter subhalo as a plausible candidate. This observation opens up the possibility that detailed studies of stellar streams could measure the mass spectrum of dark-matter substructures and illuminate the nature of dark matter.

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