Test for the gas-pressure dependence of the photophoretic velocity. a) Photophoretic velocities of individual black SiO₂ dust aggregates for two different gas pressures. The solid line shows a linear fit to the data, which, within the limits of the accuracy of the fit, shows the pressure-independence of the photophoretic velocity. b) The cumulative size distribution of the subset of dust aggregates from a) for the two pressures.