Results of the PCA analysis. The principal components PC1 and PC2 are defined in Eq. (8). Clumps with positive (resp. negative) signals are shown in full circles (resp. empty squares). Top panel: map from the PCA for positive signals. Selected (resp. rejected) clumps are shown in blue (resp. red). The blue dashed straight line shows the limit between selected and rejected clumps and follows the relation PC2 = −1  ×  PC1  +  0.55. Bottom panel: map for rejected signals: negative signals and side lobes. Side lobes, identified in Sect. 2.4, are shown in green, and the other negative signals not identified as side lobes are indicated in red. We note that side lobes can have a high S/N. Most side lobes correspond to negative signals, but we also eliminate four side lobes with a positive signal.