are removed. This step is very useful to suppress the detection of weak RFI (radio frequency interference) residuals. We adopt a detection threshold equal to 4 $\sigma$. We note that the clipping is performed on the absolute values of the voxels in the filtered data cubes, meaning that our initial source list contains sources with negative total flux. These parameter settings for SoFiA are chosen after several tests and provide a good balance between suppressing the noise and gathering sufficient statistics for the following analysis..

SoFiA calculates a reliability index $R$ for each source with positive total flux (Serra et al.2012, 2015). We chose 0.01 Jy beam$^{-1}$ as a minimum total flux and R$>$0.99 to classify a positive detection as a reliable source. We will refer to the remaining positive and negative detections (the noise peaks) as the “candidates” hereafter. While the division between reliable sources and candidates is not strict, we will see in section \[sec:matchopt\] that the candidates close to the threshold of reliable sources do not dominate the total signal in candidates.

Using these selection criteria, besides the 40 primary galaxies, SoFiA detects in total 59,757 sources from the 40 cubes, of which, using the scheme as 