Histogram of the intrinsically faintest detectable substructure in each galaxy. The intrinsic flux threshold is computed for a substructure of 20 pixels with a 2σ detection per pixel, with σ measured from the local background around each galaxy (see Sect. 3.2), multiplied by 4πD_L²/(1 + z) to take out the redshift dependence of the flux density. The vertical line marks the 5% quartile of the distribution, that is the value such that 95% of the galaxies have an intrinsic flux detection threshold that is higher than this value.