Continuum-subtracted IRAM-PdBI spectra, maps and position–velocity diagram of the CO(1–0) emission line of Mrk 273. Panels a) and b) show the spectra extracted from a circular aperture with a diameter of 5 arcsec and from the central pixel, respectively. For display purposes, the spectra are re-binned in channels of 53 km s^-1 (and in channels of 160 km s^-1 in the insets). In both panels a) and b), the narrow core of the line is fitted with two Gaussian functions (red dashed lines, further comments in the text and in the appendix). c), d) Cleaned map of the emission integrated in the blue and red-shifted CO(1–0) wings. Contours correspond to 1σ (1σ rms noise of the two maps is 0.2 mJy beam^-1). The physical scale at the redshift of the source is 0.746 kpc arcsec^-1. e), f) Position–velocity diagram along the major axis of rotation. Contours are in steps of 1σ (up to 10σ) in panel e), and 5σ in inset f); negative contours are in steps of 1σ (magenta dashed lines). Each pixel corresponds to 2.2 arcsec × 120 km s^-1 in diagram e), and to 1.1 arcsec × 54 km s^-1 in inset f).