Hypervelocity Stars: From the Galactic Center to the Halo

Hypervelocity stars (HVSs) traverse the Galaxy from the central black hole to the outer halo. We show that the Galactic potential within 200 pc acts as a high-pass filter preventing low-velocity HVSs from reaching the halo. To trace the orbits of HVSs throughout the Galaxy, we construct two forms of the potential which reasonably represent the observations in the range 5-10⁵ pc: a simple spherically symmetric model and a bulge-disk-halo model. We use the Hills mechanism (disruption of binaries by the tidal field of the central black hole) to inject HVSs into the Galaxy and to compute the observable spatial and velocity distributions of HVSs with masses in the range 0.6-4 M_☉. These distributions reflect the mass function in the Galactic center, properties of binaries in the Galactic center, and aspects of stellar evolution and the injection mechanism. For 0.6-4 M_☉ main-sequence stars, the fraction of unbound HVSs and the asymmetry of the velocity distribution for their bound counterparts increase with stellar mass. The density profiles for unbound HVSs decline with distance from the Galactic center approximately as r⁻² (but are steeper for the most massive stars, which evolve off the main sequence during their travel time from the Galactic center); the density profiles for the bound ejecta decline with distance approximately as r⁻³. In a survey with a limiting magnitude of V≲ 23, the detectability of HVSs (unbound or bound) increases with stellar mass.