The Power of Milky Way's Stellar Streams Enabled by Multi-Object Spectroscopic Surveys

We are entering an extremely data-rich era in the next decade, with full 6D+chemistry information on dozens of stellar streams, to shape our understanding on the chemo-dynamical evolution of the Milky Way, as well as the nature of the dark matter. In this talk, I will discuss two ongoing spectroscopic programs to study the stellar streams in our Milky Way and highlight a few latest scientific results from these two programs. The Southern Stellar Stream Spectroscopic Survey (S5), started in 2018, is the first systematic program pursuing a complete census of known streams in the Southern Hemisphere. The science results from S5 include a homogeneous study of the kinematic and chemical properties of dozen streams in our Milky Way, the finding of a stream at ~30 kpc possibly perturbed by the dark matter subhalo, the constraints on the mass of the Milky Way and the Large Magellanic Cloud with stellar streams, and the discovery of the fastest hyper velocity stars ejected from Galactic center that can be used to study the shape of the Milky Way halo. The Milky Way Survey of the Dark Energy Spectroscopic Instrument (DESI), on the other hand, is a recently started 5-yr spectroscopic program in the Northern Hemisphere. With just the first year of data collected in 2021-2022, 4 million unique stars have been observed by DESI, including many stars in the streams of the northern sky (e.g. GD-1) and showing some interesting features as well as the streams beyond the Milky Way.