MIPS 24 μm emission overlaid with infrared extinction contours (Kainulainen & Tan 2013) starting from A_V = 25 mag and progressing inwards in steps of 20 mag. Massive dense Herschel cores from Nguyen Luong et al. (2011) are marked with white diamonds. The white dashed line indicates the peak intensity ridge of NH₃ (1, 1) emission (see Sect. 2.1). The dashed gray rectangles illustrate the extent of IRAM 30 m (outer box) and PdBI (inner box) N₂H⁺ maps (Henshaw et al. 2013, 2014), while the extent of the whole figure corresponds to the area mapped with the GBT.