Tidal ribbons

Tidal debris from Galactic satellites generally forms one-dimensional elongated streams, since nearby Galactic orbits have almost identical frequency ratios. We show that the situation is different for orbits close to the Galactic disc, whose vertical frequency Ωz is strongly amplitude dependent. As a consequence, stars stripped from a satellite obtain a range of values for Ωz and hence of frequency ratios, and spread into two dimensions, forming a ribbon-like structure with vertical extent comparable to that of the progenitor orbit. In integrals-of-motion space, tidal ribbons are clumps, which offer the best chance of detection and allows the determination of the Galactic potential vertically across the disc.