Apparent magnitude

For each spectral group Blumenthal and Miley (1979) calculated . The limiting fluxdensities are 10 Jy for 3C and 2 Jy for 4C sources. The limiting visual magnitude was 19.4 for the 4C sources (cut-off magnitude of the Palomar Sky Survey) and for the 3C sources. This was done because the 3C radio sources are on average brighter than 4C sources. Comparison of the two samples is much better in this manner.

In figure 5 is plotted against the spectral index . The two samples show the same kind of behaviour. There is a strong correlation between apparent magnitude and spectral index. But if the spectral indices calculated at lower frequencies are plotted there is a remarkable difference. Figure 6 shows plotted against . The correlation found with is completely gone.

These differences are probably due to a combination of two effects. First, a large number of the more distant and luminous steep spectrum sources must have spectra which become optically thick at frequencies close to 100 MHz. Secondly, there is a population of less luminous relatively nearby sources which have very steep spectrum components that dominate below 178 MHz.