Contour maps obtained with LABOCA at 870 μm overlaid on the Spitzer/MIPSGAL 24 μm images for the IRDCs G329 (top), G331 (middle), and G335 (bottom). The beam HPBW (216) and a 2-pc scale-bar are shown in each panel. Contours for G329 are 3, 6, 12, 24, 48, 96, 192 times 0.037 Jy beam^-1, the rms noise of the image. Contours for G331 are 3, 6, 12, 24 times 0.044 Jy beam^-1, the rms noise of the image. Contours for G335 are 3, 6, 12, 24, 48, 96, 192 times 0.032 Jy beam^-1, the rms noise of the image. The red stars and the P labels represent the selected positions for molecular N₂H⁺ (3–2) observations.