Left: layout of the 1.6″-wide slit (white region) with the position of the quasar marked as red cross. The dashed line represents our measured constraint on ρ_‖. The value ρ_⊥ is constrained to be within the slit width. The black points show the distribution of possible locations of the emission, taking into account a normally distributed uncertainty on ρ_‖ and a uniformly distributed random angle between quasar-galaxy and slit directions. Right: probability distribution function of the impact parameter.