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

Abstract

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' ($\hat{I}\pm$ - S) for extragalactic radio sources selected at metre and decametre wavelengths, the latter for the first time. This newly determined $\hat{I}\pm$ - S${}_{340\mathrm{MHz}}$ relation shows a progressive flattening of $\hat{I}{\pm }_{\mathrm{median}}$ towards lower flux densities, starting from its steepest value (peak)occurring near S${}_{340\mathrm{MHz}}$ $\sim$ 1-2 Jy. This resolves the controversy extant in the literature since the 1980s. The $\hat{I}\pm$ - S${}_{54\mathrm{MHz}}$ relation, too, shows a spectral index flattening with decreasing flux density which, however, is significantly milder and the relation is less sharply peaked than that found at 340 MHz. A possible reason for the difference could be that the 54 MHz sample has a distinctly stronger/conspicuous presence ( at $\sim$ 20% level) of very steep spectrum sources having $\hat{I}{\pm }_{54}^{1400}<$ -1.3, most of which are probably associated with clusters of galaxies.

Accepted for Publication in A&A Letters. 4 Figures and 2 tables. Comments are welcome