Time evolution of the volume density of the spiral arm segment selected in Figure F.1 (Appendix F), shown as a function of distance along the spiral ridge. Each column represents a different snapshot, covering a 20 Myr period in 5 Myr intervals. The points are colour-coded (from top to bottom) by (i) radial velocity residuals (V_R), (ii) tangential velocity residuals (Δ – V_ϕ), (iii) velocity divergence (∇ · V), and (iv) the fraction of gas cells exceeding a critical volume density threshold of 700 cm⁻³. Negative values of ∇ · V indicate converging gas. The two spiral arm sections discussed in the main text are highlighted with different shades of grey and tracked across all times as a visual guide.