Time evolution of the luminous surface density of the primary disk material in a direct model with a large pericentre distance and mass ratio 18:1 (model M18PlDb, left panels) and in a retrograde model with a short pericentre and lower mass ratio (model M9PsRb, right panels), both assuming an initial S0b galaxy. Face-on views centered on the initial primary galaxy are plotted. Snapshots corresponding to times from 20 to 260 and from 20 to 100 are shown from top-to-bottom and left-to-right of each figure, using a time step equal to 12 and to 4 in each case (in simulation units). Scaling the primary S0b galaxy in both models to the MW (see Sect. 4), the total time period represented in each figure corresponds to ~4.5 Gyr and ~1.6 Gyr, respectively. A rainbow colour palette is used to represent different surface density levels in a logarithmic scale, with redder colours indicating higher values. Spatial scales in both axes are provided in simulation units.