Heliocentric velocity residual maps for Δ(−V_ϕ) (left) and ΔV_R (right) for Gaia DR3 data from Khanna et al. (2023) (top) and our model’s stellar (middle) and gaseous (bottom) components in the galactic mid-plane (|z| < 0.25 kpc). The stellar maps use a binning identical to those of the Gaia data, of 250 × 250 pc² pixels, for a direct comparison, while the gas maps have been binned to cells of 60 × 60 pc². The solid grey lines on the top panels represent the gaseous spiral arm tracks from Reid et al. (2019). For our simulation, the stellar and gaseous spiral arm ridges extracted from the sharpened images of Fig. C.1 are superimposed in black and grey, respectively. The position of the Sun is marked as a black circle, and the Galactic centre is outside this image, at (8.275,0) kpc.