The spectral index-flux density relation for extragalactic radio sources selected at metre and decametre wavelengths

We use the recent releases of sensitive VLA/LOFAR large-area surveys at 340 MHz and 54 MHz, in conjunction with the 1.4 GHz NVSS, to accurately determine the 'spectral index-flux density relation' (alpha-S) for extragalactic radio sources selected at metre and decametre wavelengths, with the latter explored for the first time. This newly determined alpha-S (340MHz) relation shows a progressive flattening of alpha(median) towards lower flux densities, starting from its steepest value (peak) occurring near S (340MHz) similar to 1 - 2 Jy. This resolves the controversy extant in the literature since the 1980s. The alpha-S (54MHz) relation also shows a spectral index flattening with decreasing flux density which is, however, significantly milder and the relation less sharply peaked than that found at 340 MHz. A possible reason for the difference could be that the 54MHz sample has a distinctly stronger and more conspicuous presence (at similar to 20% level) of very steep spectrum sources having alpha(1400)(54) < -1.3, most of which are probably associated with clusters of galaxies.

Automated summary

We accurately determined the 'spectral index-flux density relation' (alpha-S) for extragalactic radio sources selected at different wavelengths using recent survey data. The relation shows a progressive flattening of alpha towards lower flux densities, resolving a long-standing controversy in the literature. The relation is less sharply peaked at lower wavelengths due to the presence of very steep spectrum sources associated with clusters of galaxies.