Example of the effect of matched filtering on the detectability of a faint emission-line source in the MUSE datacube of the Hubble Deep Field South (Bacon et al. 2015). Shown is a Lyman α emitting galaxy at redshift z = 3.278 with flux F_Lyα = 1.3 × 10^-18 erg s^-1 (ID #162 in Bacon et al. 2015). The panels in the first row display four different spectral layers of the continuum-subtracted datacube F. The second row shows the same spectral layers, but for the filtered S / N cube (Eq. (9)) that was used to build the catalogue of emission line sources via thresholding (Eq. (18)). The leftmost panels show a layer significantly away from the emission line peak, the other panels show layers at spectral coordinates z_peak − 1, z_peak, and z_peak + 1, respectively, where z_peak designates the layer containing the maximum S / N value of the source. The third row shows the flux density spectrum in the spaxels at (x_peak,y_peak) and (x_peak ± 1,y_peak ± 1), where x_peak and y_peak are the spatial coordinates of the highest S/N value. The bottom row shows a S/N spectrum extracted from the S / N cube at x_peak and y_peak.