Galactic distributions for different types of HMSCs. Panel a: top view of HMSCs in the Galaxy. The red, yellow, and blue dots represent AS, PA, and NA HMSCs, respectively. The size of the dot increases with T_dust of HMSCs. The gray region represents |l| < 3°. The typical error in position is shown as a white cross. Panels b and c: latitude and longitude distributions for bubble (see Kendrew et al. 2012; Hou & Gao 2014) and different types of HMSCs. The green dashed lines in panel c indicate the tangent directions of the spiral arms, which are shown as white dashed lines in panel a.