The mass-loss history of the Sagittarius dwarf galaxy

Accretion of a dark-matter subhalo is the key process in growth of galaxies and dark-matter halos, however, orbital decay of any merger has not yet been reconstructed. In this talk I will present evidence for a spectrum of orbital resonances in a sample of nearly 20,000 Milky Way stars beyond the disk plane as nearly equidistant peaks in orbital energies, and show that the accretion of a Sagittarius-like dwarf galaxy forms such ripples. Idealized numerical experiments show that the ripple strength is set by the mass of the merger, while their positions depend on its orbital history. Matching the narrow energy spacing between the ripples requires including the effects of dynamical friction and mass loss. As the mass-loss rates of dark-matter halos depend sensitively on their density profile, the detected ripples could provide a unique constraint on the particle nature of dark matter.