Histograms showing the distribution of the a_IR coefficient values obtained for the different hybrid tracers. The parameter a_IR is computed using the expressions 16 and 17. The red dashed line corresponds to the median value of this coefficient. The red tick marks shown at the top refers to the 1σ dispersions measured as the interval that includes 68% of the data points around the median quoted in Table 3, while black tick marks indicate the standard error of the median computed from the asymptotic variance formula using these 1σ dispersions. Type-2 AGN galaxies are excluded from the histograms at right.