Modeled radio-mm spectral indices, α as a function of redshift. The solid and dashed lines correspond to and , respectively, while the green and blue hashed regions correspond to the uncertainty range due to varying dust SED temperatures spanning 25 to 60 K. The axes for the two spectral indices have been offset for clarity. Green circles and blue squares indicate those sources in our sample with spectroscopic redshifts which are detected at 3 and 1.4 GHz, respectively. AzTEC/C61 demonstrates an inverted radio spectrum and is suspected of hosting an AGN, so its extreme spectral index is not used as a redshift indicator (Miettinen et al. 2017a).