In the coming decade, thousands of stellar streams will be observed in the halos of external galaxies with the Nancy Grace Roman Space Telescope, the Euclid Space Telescope, and the Vera C. Rubin Observatory. Stellar streams form when a dwarf galaxy or a cluster of stars is torn apart due to an underlying galactic potential, leaving behind a swath of thousands of stars that exhibit coherent, ordered motion. These streams are sensitive to the distribution of dark matter and to the population of dark matter subhalos in galaxies, both of which depend on the mass and interactions of the dark matter particle. In this talk, I discuss how to use the incoming wealth of stellar stream data to rule out dark matter candidates. I first focus on dwarf streams and present new models of the Centaurus A (Cen A) dwarf companion Dwarf 3 (Dw3) and Dw3's associated stellar stream. With a novel external galaxy stream-fitting technique, I show that there are many viable stream models that fit the data well, provided that Cen A has a dark matter halo mass larger than M_200 > 4.7 x 10^12 Msun. I also demonstrate that just one radial velocity measurement breaks degeneracies between stream morphology and dark matter halo mass. In the second part of the talk, I discuss stellar streams from globular clusters. Due to their low velocity dispersions, these streams are sensitive to gravitational interactions with low-mass dark matter subhalos. In the Milky Way, we know of a handful of stellar streams with noticeable under-densities, however, the Galactic bar, molecular clouds, and spiral arms can also lead to similar signatures in the streams. If we can instead find globular cluster streams in external galaxies without these baryonic perturbers, gaps in such streams can be easier to decipher and serve as a test of LCDM. I present the Hough Stream Spotter code which can rapidly and systematically search for linear structures in external galaxies. The Hough Stream Spotter combined with the Nancy Grace Roman Space Telescope will find hundreds of thin globular cluster streams in external galaxies. Lastly, I will discuss how to use stellar streams as tools to rule out dark matter candidates that are inconsistent with the new wealth of data.
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