Far-infrared to radio correlation. Sources in our sample with extremely flat low-frequency radio spectral indices are plotted with light-coloured squares, and the positions of the ‘normal’ sources are plotted with dark circles. The FIRC relation calculated from our data following Eq. (1) is plotted as a dark dashed line, while the shaded region shows the 1σ and 3σ variances, respectively. Data from Ivison et al. (2010) are plotted in green crosses for comparison, with the FIRC from that work plotted as a dotted green line.