Plot of the N₂H⁺ linewidth versus the dust column density. Marked by green dots are pixels that lie within IRDCs. Overplotted with red Xs are all PACS sources that were mapped. Blue dots also have a 24 μm detection, while the pale blue dots represent sources that are saturated at 24 μm. The uncertainties given for IRDC 18223 are representative for all regions. For IRDC 18454 the linewidths were multiplied by a factor of 0.5 to fit the data points into the plotting range.