Spectral index distribution. Distributions between 1.4 and 22 GHz (top) and between 22 and 43 GHz (bottom). The red solid line represents the detected galaxies at 22 (top) and at 43 GHz (bottom). Black dotted lines represent non-detected galaxies. The arrows in each panel indicate the lower limit of spectral indices for non-detected galaxies. Galaxies can be divided into three groups according to spectral index: steep (α > 0.5), flat (0 < α < 0.5), or inverted (α < 0) spectra.