Distribution of background column densities for the L1688 (orange) and L1689 (green) prestellar cores. A clear peak in column density is visible, suggestive of a column-density threshold for prestellar core formation. However, this peak lies at a significantly higher column density in L1688 (orange) and in L1689 (green) than in other Gould Belt regionssuch as Aquila (cf. Könyves et al. 2015). The vertical dashed line marks the same fiducial threshold at as in Fig. 15.