Centroid velocity maps derived from pixel-per-pixel fitting of the N₂ D⁺ ( top left) and pNH₂D (top right) spectra with pyspeckit (cf. Section 4). The white contours at 0.8, 1.7, and 2.5 × 10²² cm⁻² trace the gas column density as derived from Herschel data (from Spezzano et al. 2017) and are the same on both panels. The white filled circles show the 23″.5 beam size (0.019 pc at the 170pc distance of L1544) and the white vertical lines indicate the 0.05 pc scale. The excitation temperature maps are shown in Appendix A. The bottom plots show the distribution of the velocity for N₂D⁺ in red and pNH₂D in blue (bottom left) and a scatter plot (bottom right). The median values of the distributions indicated by the vertical dashed lines are 7.16 km/s for N₂D⁺ and 7.21 km/s for pNH₂D. The oblique solid and dashed blue lines indicate the 1:1 and the −0.05 km/s relations, respectively. The gray error bars on the scatter plot are the ±1σ uncertainties on the centroid velocities derived from the pyspeckit fit. The mean values of these uncertainties are 0.003 and 0.006 km/s for the N₂D⁺ and pNH₂D data points, respectively (the histograms of the uncertainties are presented in Appendix A).