Fraction of HMSCs. The fraction is calculated by the number ratio of HMSCs to massive ATLASGAL clumps in each distance range, as shown in the numbers in figure. The sample of ATLASGAL clumps is taken from the catalog in Csengeri et al. (2014) and the clump distance is taken from Urquhart et al. (2018). ATLASGAL clumps with a 870 μm peak intensity higher than 0.5 Jy beam⁻¹ are extracted to be in line with the column density criterion for massive star formation in Sect. 2.1. The black, red, and blue lines indicate the changes of fractions with distance.