Integrated emission of stacked Hnα (n = 151–158; angular resolution 16.′′8 × 13.′′8, 0.85 pc). The color scale shows velocity integrated intensity (between −40 and 60 km s−1). Contours (white) show 870 μm emission (ATLASGAL survey; Schuller et al. 2009; contours as in Fig. 1). Crosses indicate infrared star clusters (Alves & Homeier 2003). The positions of the spectra shown in Fig. 3 are marked, as well as the locations of the 3.6 cm continuum sources “M”, “L”, “J” and “JJ” (De Pree et al. 1997), which are mentioned in Sects. 3.1 and 5.1 (“x”-shaped symbols).