Stacking S/N maps for the TNG50 galaxy 55013. First column: stacking regions where S/N > 5 (blue squares) overlaid with the grey-scale noiseless moment 0 map, blanked from the emission already detected by SoFiA-2. Top to bottom is for cell sizes of 14 × 14, 9 × 9 and 5 × 5 beams, respectively. Second column: the integrated flux of the signal for stacking regions where the HI column density is > 3.6 × 10¹⁷ cm⁻². The blue contours enclose the cells where S/N > 5. Third column: the noise of the stacked spectrum multiplied by the square root of the number of channels covered by the source. Fourth column: integrated S/N of the signal in the stacked spectrum for the regions. The S/N = 5 level is indicated also on the colourbar.