Ratio (top panel) and the sum of the surface mass densities (bottom panel) related to the second and first infall events as a function of Galactocentric distance R [kpc]. The dashed line in the top panel represents the global fitted value, and the shaded area is the related uncertainty. Note: the errors associated with the total regional surface mass densities are calculated as described in the text. Results from S21 are denoted in green.