Continuum and line images of the IRDC 18223 filament. The left and 2nd panel show the Spitzer 24 μm emission in color with a stretch to increase the visibility of the mid-infrared dark filament. The left panel presents in contours the 3.2 mm continuum emission with natural weighting and the same levels as in Fig. 2. For comparison, the 2nd panel shows in contours the integrated N₂H⁺(1−0) emission covering all hyperfine structure components between 35 and 55 km s^-1. The third and 4th panel present the 1st and 2nd moment maps (intensity-weighted peak velocities and line-width) extracted from the isolated most negative hyperfine structure component. In the 3rd panel, the velocities are shifted by the separation of the hyperfine structure line of 8 km s^-1 to the v_lsr. The contours in the 3rd and 4th panel both show the same continuum contours as in the left panel. The black line in panel 3 gives the direction of the position-velocity cut shown in Fig. 6. A scale-bar is depicted in the left panel.