Redshift versus spectral index plot for all sources with spectroscopic redshift in our sample.Sources with an actual measurement of α are split in spectroscopic classes (AGN: black square, SF: black star, unknown: black diamond). Sources with an upper limit on α only (black dots) were not split up. Note that R² = 0.09 for the fit to our data points (black symbols) indicates highly uncorrelated quantities. A slight correlation is usually assumed when R² > 0.5 with good correlation for R² > 0.85.