NGC 2403 moment maps. Left panel: primary-beam-corrected H I column-density map of NGC 2403 with reversed grey-scale colour map from faint (grey) to bright (black) emission. The sharp cut-off in the bottom left is due to the 20% cut-off threshold of the primary beam response, which is illustrated with the light grey curve. The contours levels are (5, 75, 200, 500, 1000, 2000) × 10¹⁸ cm⁻². The column density limit 3 × σ × dv = 2 × 10¹⁸ cm⁻², where dv = 5.3 km s⁻¹ is the spectral resolution, is reported at the bottom. In the bottom right we show the 52″ × 49″ beam. Central panel: intensity-weighted mean velocity field of NGC 2403 with respect to the systemic velocity of 133.2 km s⁻¹, given at the bottom. Dashed contours (−105, −70, −35) km s⁻¹ refer to the approaching side, while solid lines correspond to radial velocities of (35, 70, 105) km s⁻¹. The thick grey line is the kinematical minor axis. Right panel: second moment map, where the signature of the filaments with their anomalous velocities is visible in the increased second-moment values.