Correlations between colour ratios and 3.6 μm, tracing the stellar population (left), and 24 μm, tracing star formation regions (right). The best fits of the form log ₁₀(y) = Alog ₁₀(x) + B are shown by the solid lines. The correlation with 3.6 μm emission becomes less significant as the frequencies of the colour ratios increase, whilst the 24 μm correlation becomes more significant. The correlation values are given in Table 2.