What are dense patches of sources that crop up in out catalogues?

We take a re-run of tract 9615 and look at the r-band deepCoadd forced src catalogue. Images below are r-band when monochrome, and gri when RGB. We will use these data to determine dense regions in the catalogue, performing an n-th nearest neighbour match. Having identified suitably clumped regions, we will take a closer look to see what's going on.