Histograms of the spectral index distribution. Upper panel: spectral index distribution of all compact GLOSTAR sources with Y-factor < 2 and a counterpart detected at 1.5 GHz in the THOR survey. The red and yellow hatched histograms show the distribution of compact radio sources that are associated with mid-infrared sources and dust emission, respectively. Lower panel: distribution of the associated uncertainties to the spectral index measurements. The bin sizes used in the upper and lower panels are 0.1 and 0.02, respectively.