Comparison of Hi-GAL source surface density (histogram and right y-axis) with H₂ gas surface density (dashed line and left y-axis from Blitz 1996) in galactocentric radius. The ratio between the two quantities is a proxy for star-formation efficiency: the more H₂ gas per Hi-GAL source, the less efficient is the star formation. From the plot it is possible to estimate that in the outer Galaxy there is about 100 times more gas per Hi-GAL source than in the inner Galaxy (galactic ring, around 4−5 kpc from the Galactic centre), supporting the idea that star formation is more efficient in the inner Galaxy.