Velocity structure. Mean velocity maps of (a) the F₁F = 2,3–1,2 component of N₂H⁺ J = 1–0 and (b) C¹⁸O J = 1–0 calculated within V_LSR = [6.75 km s⁻¹, 7.50 km s⁻¹] and [6.0 km s⁻¹, 7.5 km s⁻¹], respectively. Contours from the 850 μm image are overlaid. The rest of the symbols in the panels a and b have the same meaning as in Fig. 2. (c) Model vertical rotation velocities, V_rot, against radius, Δr, adopted in the radiative transfer model. (d) Central velocities and their error bars of the spectra along the main vertical cuts. Two y-axes are vertical offsets to the center and declination, respectively.