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 $\Omega_z$ is strongly amplitude dependent. As a consequence, stars stripped from a satellite obtain a range of values for $\Omega_z$ 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 offers the best chance of detection and allows the determination of the Galactic potential vertically across the disc.