Detection Significance Index (DSI). Top: image showing an aperture of 74.41 arcsec² (net area excluding pixels identified as background or foreground sources) placed on a faint segment of the stream around PGC 597851; bottom: histogram showing the flux distribution in 10 000 apertures of the same size, randomly placed over non-detection regions of the image. The green vertical line shows the flux in the green aperture above, with a measured surface brightness of 26.91 mag arcsec⁻², at 3.73σ (DSI) from the distribution median. Note that the histogram is not symmetric, as different regions can have different depths.