Spectral index distributions for the SUMSS 843 MHz sources cross-matched to one or more XXL-S 2.1 GHz sources. The black line represents the 699 SUMSS sources cross-matched to all ATCA XXL-S sources within 52.8″ and the red line represents the 560 SUMSS sources that were matched to exactly one ATCA XXL-S point source. The median spectral indices for these cases are − 0.83 and − 0.85, respectively.