Number of sources with a given CO(7–6)/FIR luminosity ratio, colour-coded as in Fig. 2. Data are taken from Kamenetzky et al. (2014). Values for the lens ratios show a range containing the observed values Zhang et al. (in prep.) and an uncertainty of a factor of 10, likely comprehensive of the unknown effect of the gravitational lensing effect. For comparison, the value of the Galaxy centre, and inner and outer Galaxy are also shown (Fixsen et al. 1999).