Density distribution of young (<30 Myr; left) and older (>150 Myr) sources. In the left panel, star forming regions are indicated. The solid contour lines represent the density distribution of all sources (Fig. 10, left) and are computed as described in Sect. 4. The labels highlight some of the more prominent OB associations (their locations are from Table 1 of Wright 2020). The solid lines indicate constant Galactic longitudes.