Spectral index, , distribution of sources matched between 1.4 GHz and 153 MHz (grey points). The difference in resolution is 45″ (NVSS) and 25″ (GMRT) and multiple GMRT matches to a single NVSS source have been merged into one. The vertical line shows 5σ_avg, where σ_avg is the average rms noise in the GMRT mosaic. The diagonal dotted line indicates the incompleteness limit due to the sensitivity of NVSS and sources with upper limits are plotted as black points along this line. The horizontal dashed line shows the KM mean spectral index of −0.87 ± 0.01 accounting for upper limits. The large black triangles show the mean spectral index in 5 logarithmic bins. Error bars on individual points are not plotted for clarity, but a single bar in the top right indicates the maximum and minumum errors in the dataset.