Velocity structure of NGC 7538 IRS 1. Contours: observed dust continuum map, contours are 3σ to 21σ in steps of 6σ, where σ = 0.025 Jy beam^-1. First column: order 1 moment maps, i.e. velocity structure. The common scale is shown at the top of the column. Second to fifth columns: channel maps with the velocity labeled at the top. Rows a and b: observed H¹³CO⁺ 4−3 a) and C¹⁷O 3−2 b) maps. The grayscale for the channel maps is shown in the righthand side of the figure. Rows c and d: synthetic maps generated with RATPACKS (Sect. 4.1.1) for a logarithmic spiral with radial expanding motions c) and with rotational motions d).