Source spectral indices between 150 and 220 GHz versus the spectral indices between 95 and 150 GHz. Note that 80 sources are omitted from the plots because their fluxes are below 0 and filled-in symbols indicate sources inside the EDF-S. Left panel : spectral indices that are color-coded according to their 150 GHz flux values: 0.4, 1.2, 2.0, 2.7, and 3.9 mJy are roughly equivalent to 1, 3, 5, 7, and 10σ detections. This view highlights the population separation between high signal-to-noise sources, which typically have synchrotron-dominated spectra, and dim sources which comprise the dusty population. Right panel : spectral indices that are represented by colors and symbols according to their external catalog associations (see the caption of Fig. 5). In this view, we emphasize that sources with dust-dominated spectra are uniquely discovered by low-noise millimeter-wavelength surveys such as those conducted by the SPT. The top and right panels are the kernel density estimations of each α axis, showing the bimodal distributions in both ${\alpha }_{150}^{95}$ and ${\alpha }_{220}^{150}$ but especially how ${\alpha }_{220}^{150}$ can be used to assign a spectral type of synchrotron or dusty; in this analysis, we adopted the cutoff value of ${\alpha }_{220}^{150}=1.5$ = 1.5 (marked by dotted lines).