Histograms of spectral indices using the values reported in Table 5. The dashed vertical line denotes α = − 0.1, which is thelowest possible value for thermal emission. Top: between fluxes at 150 and 325 MHz; when only an UL to the flux at 150 MHz was available, we also estimated a lower limit (LL) to the spectral index. Bottom: between fluxes at 325 and 610 MHz; when only an UL to the flux at 610 MHz was available, we also estimated an UL to the spectral index.