Using frequency maps to explore the distribution function of the Milky Way stellar halo

Abstract

Resolved surveys of the Milky Way's stellar halo can obtain all 6 phase space coordinates of tens of thousands of individual stars, making it possible to compute their 3-dimensional orbits. Spectral analysis of large numbers of halo orbits can be used to construct frequency maps which are a compact, yet informative representation of their phase space distribution function (DF). Such maps can be used to infer the major types of orbit families that constitute the DF of stellar halo and their relative abundances. The structure of the frequency maps, especially the resonant orbits, reflects the formation history and shape of the dark matter potential and its orientation relative to the disk. The application of frequency analysis to cosmological hydrodynamic simulations of disk galaxies shows that the orbital families occupied by halo stars and dark matter particles are very similar, implying that stellar halo orbits can be used to constrain the DF of the dark matter halo, possibly impacting future direct dark matter detection experiments. An application of these methods to a sample of \sim 16,000 Milky Way halo and thick disk stars from the SDSS-SEGUE survey yields a frequency map with strong evidence for resonant trapping of halo stars by the Milky Way disk, in a manner predicted by controlled simulations in which the disk grows adiabatically. The application of frequency analysis methods to current and future phase space data for Milky Way halo stars will provide new insights into the formation history of the dierent components of the Galaxy and the DF of the halo.