The distribution of the density (top) and the ratio of the cooling time to the evolution time (bottom) in the r−z plane at ϕ = 0 at t ≃ 10,000, 20,000, 30,000, 40,000 s. The location of the thermalization photosphere as seen by a distant observer is plotted using a white line. We define the thermalization optical depth as ﹩\sqrt{{\tau }_{{\rm{t}}}{\tau }_{\mathrm{ff}}}﹩, for τ _T (τ _ff) the Thomson (absorption) optical depths integrated radially inwards from the outer boundary. The prominent break in structure roughly halfway out in the debris corresponds to the density peak of the expanding ring.