Radio spectral index between 150 MHz and 1.4 GHz defined as F_ν ∝ ν^−α versus source luminosity at 150 MHz. The green circles are the lower limits derived for the five extended sources not detected at 1.4 GHz. The remaining extended sources in the LOFAR images are marked with blue squares, while the black dots are the unresolved ones. The smaller red dots represent lower limits due to the lack of a detection at 1.4 GHz.