Stray, swing and scatter: Angular momentum evolution of orbits and streams in aspherical potentials

Change log
Erkal, D 
Sanders, JL 

In aspherical potentials orbital planes continuously evolve. The gravitational torques impel the angular momentum vector to precess, that is to slowly stray around the symmetry axis, and nutate, i.e. swing up and down periodically in the perpendicular direction. This familiar orbital pole motion

  • if detected and measured - can reveal the shape of the underlying gravitational potential, the quantity only crudely gauged in the Galaxy so far. Here we demonstrate that the debris poles of stellar tidal streams show a very similar straying and swinging behavior, and give analytic expressions to link the amplitude and the frequency of the pole evolution to the flattening of the dark matter distribution. While these results are derived for near-circular orbits, we show they are also valid for eccentric orbits. Most importantly, we explain how the differential orbital plane precession leads to the broadening of the stream and show that streams on polar orbits ought to scatter faster. We provide expressions for the stream width evolution as a function of the axisymmetric potential flattening and the angle from the symmetry plane and prove that our models are in good agreement with streams produced in N-body simulations. Interestingly, the same intuition applies to streams whose progenitors are on short or long-axis loops in a triaxial potential. Finally, we present a compilation of the Galactic cold stream data, and discuss how the simple picture developed here, along with stream modelling, can be used to constrain the symmetry axes and flattening of the Milky Way.

galaxies: haloes, galaxies: structure, dark matter
Journal Title
Monthly Notices of the Royal Astronomical Society
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Oxford University Press (OUP)
Science and Technology Facilities Council (ST/K000985/1)
Science and Technology Facilities Council (ST/N000927/1)
The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement no. 308024. Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy Office of Science.