Letter to the Editor

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

¹ Instituto de Astrofísica de Canarias, Calle Vía Láctea, s/n, 38205 La Laguna, Tenerife, Spain
e-mail: pdabhade@iac.es
² Universidad de La Laguna (ULL), Departamento de Astrofisica, 38206 Tenerife, Spain
³ UM-DAE Centre for Excellence in Basic Sciences (CEBS), Vidyanagari, Mumbai 400098, India

Received: 4 April 2023
Accepted: 19 June 2023

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’ (α − S) for extragalactic radio sources selected at metre and decametre wavelengths, with the latter explored for the first time. This newly determined α − S_340 MHz relation shows a progressive flattening of α_median towards lower flux densities, starting from its steepest value (peak) occurring near S_340 MHz ∼ 1 − 2 Jy. This resolves the controversy extant in the literature since the 1980s. The α − S_54 MHz 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 54 MHz sample has a distinctly stronger and more conspicuous presence ( at ∼20% level) of very steep spectrum sources having α₅₄¹⁴⁰⁰ < −1.3, most of which are probably associated with clusters of galaxies.