Continuum images of the W31 region. The shown wavelengths are marked in each panel. The 875 μm data are contoured in 3σ steps from 3 to 12σ and continue in 15σ steps from 15σ onwards (1σ ≈ 70 mJy beam^-1). The 21 cm image is contoured in 3σ steps from 3 to 12σ (1σ ≈ 5 mJy beam^-1), continuing in 0.24 Jy beam^-1 steps from 0.12 to 0.84 beam^-1, and then go on in 0.48 beam^-1 steps. The 24 μm map is saturated toward the peak positions. In the top-left panel the 3-pointed stars mark the position of an O-star by (Bik et al. 2005) and the approximate center of the southern infrared-cluster discussed by Blum et al. (2001). The two white 5-pointed stars show the positions of UCHii regions (Wood & Churchwell 1989). The ellipse marks the emission that velocity-wise is associated with cloud complexes outside our field of view (Sect. 3.2). A scalebar is shown in the top-left panel. The 875 μm and 21 cm beam sizes are shown in the top-right corners of the top-left and bottom-left panels, respectively. In the bottom-left panel, the triangles and stars show IRAC-identified protostellar class 0/I (the two classes are combined) and class II candidates, respectively, following Allen et al. (2004); Megeath et al. (2004); Qiu et al. (2008).