Spacetime diagrams showing the solid surface density Σ_solid as a function of the radial coordinate x and simulation time τ_f = 0.003 and τ_f = 0.010 particles. The surface density is shown by color, using the same color scale as in Figs. 4 and 5. The figure shows both 128 × 128 simulations and the corresponding 256 × 256 simulations. In all simulations Δ = 0.05. The higher-resolution runs produce results consistent with the 128 × 128 runs.