Probing the shape and history of the Milky Way halo with orbital spectral analysis

The phase space coordinates of individual halo stars obtained by Galactic surveys enable the computation of their full 3-dimensional orbits. Spectral analysis of halo orbits can be used to construct frequency maps which provide a compact representation of the 6-dimensional phase space distribution function. Frequency maps identify important major orbit families, and the orbital abundances reflect the shape and orientation of the dark matter halo relative to the disk. We apply spectral analysis to halo orbits in a series of controlled simulations of disk galaxies. Although the shape of the simulated halo varies with radius, frequency maps of local samples of halo orbits confined to the inner halo contain most of the information about the global shape of the halo and its major orbit families. Quiescent or adiabatic disk formation results in significant trapping of halo orbits in resonant orbit families (i.e. orbits with commensurable frequencies). If a good estimate of the Galactic potential in the inner halo (within ~50kpc) is available, the appearance of strong, stable resonances in frequency maps of halo orbits will allow us to determine the degree of resonant trapping induced by the disk potential. The locations and strengths of these resonant families are determined both by the global shape of the halo and its distribution function. Identification of such resonances in the Milky Ways stellar halo would therefore provide evidence of an extended period of adiabatic disk growth. If the Galactic potential is not known exactly, a measure of the diffusion rate of large sample of 10^4 halo orbits can help distinguish between the true potential and an incorrect potential. The orbital spectral analysis methods described in this paper provide a strong complementarity to existing methods for constraining the potential of the Milky Way halo and its stellar distribution function (ABRIDGED).