Distribution in Galactic coordinates of the sources in quality classes 0 and 1 (upper left) and 2 (upper right) of the Gaia UCD catalogue using the Aitoff projection. Lower-left panel: clusters at level 4 of the hierarchy with more than ten members identified by HMAC in the set of sources in quality classes 0 and 1. There are 88 such clusters, the largest 13 of which are identified with prominent colours and the rest plotted in salmon for clarity. Lower-right panel: rest of the sources in clusters with ten members or fewer. We mark in red and blue the two most prominent overdensities.