Top panel (bottom):: gas density profile (solid line) and gas density profile (plus symbols) at different times (reported in the legend). The profiles have been recentred and shifted horizontally by −0.12 cm, 0, 0.12 cm, and 0.24 cm for t = 0, 1, 9, and 100 s, respectively. Top panel (top):: relative difference between the gas and gas tracer density profiles in units of the expected noise level . Bottom panel: evolution of the spatial extent of an advected overdensity as a function of time for the gas (dashed) and the gas tracer particles (dot symbols) for a high-resolution run (blue) and a low-resolution run (orange, see text for details). The difference shows no spatial dependence. The gas tracers diffuse exactly as the gas.