Radial distribution of the azimuthally-averaged surface density of dense molecular gas, derived from C¹⁸O observations. We also include the azimuthally-averaged surface density distribution of the total H₂ gas derived from ¹²CO, ¹³CO, and [C ii] observations which also includes a contribution from diffuse molecular gas. All gas surface densities have been corrected to account for the contribution of He.