Continuum-subtracted IRAM-PdBI spectra and position–velocity diagram of the CO(1–0) emission line of I Zw 1. Panels a) and b) show the spectra extracted from a circular aperture with a diameter of 10 arcsec and from the central pixel, respectively. For display purposes, the spectra are re-binned in channels of 54 km s^-1. No evidence of broad wings is observed. c) Position–velocity diagram along the major axis of rotation. Contours correspond to: 1σ, 3σ, 6σ, 12σ, 18σ, 30σ, 36σ, 39σ, 42σ, 48σ; negative contours are in steps of 1σ (magenta dashed lines). Each pixel in diagram c) corresponds to 1.2 arcsec × 27 km s^-1. The physical scale at the redshift of the source is 1.175 kpc arcsec^-1.