Radio spectral index α as a function of the 3.6 μm flux density (top left), α as a function of the radio flux density S_{1.4 GHz} (top right), α as a function of the radio-to-IR flux density ratio S_{1.4 GHz}/S_{3.6 μm} (bottom left), and S_{1.4 GHz}/S_{3.6 μm} as a function of S_{1.4 GHz} (bottom right) for our sample of 34 IFRS. The symbols distinguish subsamples of different turnover properties. Green dots represent sources with a detected turnover. Sources with a possible peak are shown by purple squares, and sources without a peak by black crosses. The classification is described in Sect. 3 and summarised for individual IFRS in Table 3. Upper and lower limits are represented by arrows. The grey horizontal and vertical lines indicate the medians or limits of the median.