About twenty years ago little was known about how steep spectrum sources relate
to objects having "normal" ()
or flat radio spectra. Looking at figure 2 you see that a lot of sources have
a spectral index between -0.6 and -1.0.
There are some very flat objects and about of all extragalactic sources
observed at low frequencies (
MHz) have spectra with indices
). Baldwin and Scott (1973) and Slingo (1974 a,b) who investigated a
sample of 4C sources with steep spectra at meter wavelengths concluded that
these sources occur exclusively in rich clusters of galaxies and that they have
evolved from powerful double radio galaxies.
Tielens et al. (1978) made observations with the Westerbork Synthesis Radio Telescope (WSRT) of a sample of 4C sources with steep spectra at decimeter wavelenghts (178 to 4995 MHz). One of their conclusions was concerned with the fact that over the range of spectral index from -0.7 to -1.2 they observed a decrease of identification fraction by a factor of at least 3 coupled with a decrease in angular size by a factor of 1.5. This could be explained if steeper decimeter-wave spectrum sources were on average more luminous and located further away than sources with normal radio spectra. Further investigation was needed to improve the statistics for very steep spectrum sources.
