The H i  emission at 30′′ resolution, after the subtraction of the main body. Left: B-band image overlaid with the total H i  map. Contours are at 2.4, 4.8, 9.6, 19.2 × 10¹⁹ atoms cm^-2. Middle: velocity field. Contours range from 735.8 to 767 km s^-1 with steps of 10.4 km s^-1. The cross shows the position of I Zw 18 A. The dashed line shows the path followed to obtain the position–velocity diagram. Right: position–velocity diagram. Contours are at  − 1.5 (dashed), 1.5, 3, 6, 12σ, where σ = 0.3 mJy/beam. The vertical line corresponds to the cross in the velocity field. The star shows the Hα velocity and the spatial position of I Zw 18 C.