Tidal debris morphology and the orbits of satellite galaxies

How do galaxies move relative to one another? While we can examine the motion of dark matter subhaloes around their hosts in simulations of structure formation, determining the orbits of satellites around their parent galaxies from observations is impossible except for a small number of nearby cases. In this work, we outline a novel approach to probing the orbital distributions of infalling satellite galaxies using the morphology of tidal debris structures. It has long been understood that the destruction of satellites on near-radial orbits tends to lead to the formation of shells of debris, while those on less eccentric orbits produce tidal streams. We combine an understanding of the scaling relations governing the orbital properties of debris with a simple model of how these orbits phase-mix over time to produce a 'morphology metric' that more rigorously quantifies the conditions required for shells to be apparent in debris structures as a function of the satellite's mass and orbit and the interaction time. Using this metric we demonstrate how differences in orbit distributions can alter the relative frequency of shells and stream structures observed around galaxies. These experiments suggest that more detailed modelling and careful comparisons with current and future surveys of low surface brightness features around nearby galaxies should be capable of actually constraining orbital distributions and provide new insights into our understanding of structure formation.