Comparison of our ¹²CO/¹³CO line ratio with previous measurements in nearby galaxies. The hatched histogram displayed the ¹²CO/¹³CO line ratios with available morphological types. The Li et al. (2011), Matsushita et al. (2010) and Tan et al. (2011) samples measured the (1–0) transition and have been corrected by a factor 1.33 as discussed in the text. The Krips et al. (2010) sample and our measurements were directly measured the (2–1) transition. The black full line histogram gathers all the four samples corrected to (2–1). The black thick line histogram in the bottom corresponds to the measurements performed in this paper. The average ¹²CO/¹³CO detected in this paper tends to be a bit larger than the main distribution from other areas and other galaxies (but well below the standard isotopic ratio of 53Wilson & Rood 1994). The largest value above 50 might corresponds to an optically thin cloud, but it is affected by a large error bar (30%, see Table A.1).