The corotation gap in the Galactic HI distribution

We used the HI data from the LAB survey to map the ring-shaped gap in HI density which lies slightly outside the solar circle. Adopting $R_0$= 7.5 kpc, we find and average radius of 8.3 kpc and an average gap width of 0.8 kpc. The characteristics of the HI gap correspond closely to the expected ones, as predicted by theory and by numerical simulations of the gas flow near the corotation resonance.