  We study the behaviour of interacting self-propelled particles, whose
self-propulsion speed decreases with their local density. By combining direct
simulations of the microscopic model with an analysis of the hydrodynamic
equations obtained by explicitly coarse graining the model, we show that
interactions lead generically to the formation of a host of patterns, including
moving clumps, active lanes and asters. This general mechanism could explain
many of the patterns seen in recent experiments and simulations.
