Relative strengths of both the narrow core (black) and the broad wings (blue and red) of Mrk 231’s CO emission lines. The ratios of the CO integrated fluxes were calculated using the data reported in the upper part of Table 2. The CO spectral line energy distribution (SLED) of Mrk 231 obtained by Papadopoulos et al. (2007) is shown for comparison, along with the CO SLEDs of the Milky Way (Fixsen et al. 1999), M 82 (Weiß et al. 2005) and NGC 1266 (Alatalo et al. 2011).