Position of the IRDCs in the Galaxy (Courtesy of NASA/JPL-Caltech): G336.9, G337.1, and G337.2 are indicated by a blue circle, G010.4 and G018.7 by a red and a green one. The position of the Sun is also indicated.

Black: HIFI spectra of the para-H₂O ground-state absorption line at 1113.343 GHz for the line of sight towards G010.4 (top left), G018.7 (top right), G336.9 (bottom left), and G337.2 (bottom right). Green: FLASH CO(3−2) spectra, rescaled dividing by 50 and shifted by the amount indicated in green on the left. Red: C¹⁷O(3−2) spectrum, rescaled dividing by 16 and shifted by the amount indicated in red on the left. The tangential velocity (± 10 km s^-1, dashed) is indicated in blue.

GLIMPSE false-colour images (red: 8 μm, green: 4.5 μm, blue: 3.6 μm) of G010.4 a), G018.7 b), G336.9 c), G337.2 d). The yellow contours show the 870 μm emission from the ATLASGAL survey, ranging from 0.14 to 1.59 Jy beam^-1 in equal log steps of 0.15 dex in a); from 0.14 to 3.98 Jy beam^-1 in equal log steps of 0.16 dex in b); from 0.14 to 1.12 Jy beam^-1 in equal log steps of 0.15 dex in c); and from 0.15 to 1.91 Jy beam^-1 in equal log steps of 0.16 dex in d). The red plus indicates the position of the molecular-line observations.

High-density tracers observed with Mopra. The source name is indicated above each panel. For each source, the HCO⁺(1−0) (black) and H¹³CO⁺(1−0) (red) spectra are shown in the top sub-panel, whereas the HNC(1−0) (black) and HN¹³C(1−0) (red) spectra are shown in the bottom sub-panel. The V_LSR of the source is indicated in blue.

MALT90 HCO⁺(1−0) zeroeth moment map (contours) superimposed on the ATLASGAL sub-mm continuum emission at 870 μm (colourscale) for G010.4 (top left), G336.9 (top right), G337.1 (bottom left), and G337.2 (bottom right). The Mopra and APEX beam sizes are indicated.