The Methodology of Modelling the Milky Way with Stellar Streams

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Deg, Nathan
Milky Way , Astrophysics
In this thesis we examine the methodology of modelling the Milky Way using stellar streams. Using the orbit-fitting method we show that combining constraints from the Sagittarius stream with a suite of other photometric and kinematic observations of the Milky Way suggests a triaxial dark matter halo with axis ratios $3.3\pm 0.7$ and $2.7\pm 0.4$ where the short axis is pointed near the Sun-Galactic center line and the intermediate axis is perpendicular to the disk. Additionally, this combination of constraints reduces the uncertainty in the spherically averaged mass profile by a factor of two. Through the use of mock data we show that the combination of stellar streams with suites of other, non-stream, observations is necessary to obtain the best constraints on Galactic parameters. Suites of constraints that do not include streams lead to degeneracies in the inferred halo parameters, which the addition of a GD-1-like stream reduces. Promisingly ,the combination of two streams provides similar improvements in the inferred halo parameters as a single long stream. In addition to these studies we explore the orbit-fitting and the streakline methods of modelling stellar streams. We find that the difference between streams and the orbit of the progenitor causes the orbit-fitting method to fail to recover the generative model in most test cases. The streakline method of citet{Varghese2011} and \citet{Kupper2012} is a great improvement on the orbit-fitting method, but it also fails to recover quantities like the local circular speed for streams generated from particularly massive progenitors. Based on these results, we develop a modification of the streakline method called the distribution method. This method accounts for the stripping of stream stars at the approximate location of the inner and outer Lagrange points with different Galactic velocities than the progenitor. It provides a marginal improvement on the streakline method and consistently recovers the circular speed at the location of the observer. The distribution method holds a great deal of promise for future studies of the Milky Way involving stellar streams.
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