Continuum flux density at the peak of emission as derived from each spectral window across the surveyed frequency range (red dots). We note that extended emission in our data might account for up to ∼20% higher fluxes (Sect. 4.1). A fit to the data is shown by a continuous blue line, which is the combination of the free-free emission (dotted blue almost horizontal line) and the graybody emission (dashed blue line). See text for further details on parameters used. As a reference to illustrate the deviation from pure black body emission, that for a T_d = 50 K and 0.5″ source size is shown as a green dot-dashed line.