Comparison between Q_approx (Eq. (23)) and the median value of Q(R) (Eq. (1)). The different symbols indicate the gas tracer, with circles for HI, stars for CO, and diamonds for [CII]. The symbols are coloured according to the mass fraction of cold gas with respect to baryons and the numbers indicate the galaxy ID (see Table 1). The areas in shaded red and hatched show the instability regions for Q < 1 and Q_approx < 1, respectively. The grey solid lines represent the different ratios between the values on y and x axes. This figure shows that Eq. (23) strongly underestimate Q.