The corotation gap in the Galactic H i distribution

We used the H i data from the LAB Survey to map the ring-shaped gap in H i density that lies slightly outside the solar circle. Adopting R0= 7.5 kpc, we find an average gap radius of 8.3 kpc and an average gap width of 0.8 kpc. The characteristics of the H i gap correspond closely to the expected ones, as predicted by theory and by numerical simulations of the gas flow near the corotation resonance.