Spectral energy distributions (SED) of the two optically-dark galaxies AGS4 and AGS17. The flux densities from 100 to 500 μm are from GOODS-HerschelElbaz et al. (2011). AGS4 (top) (z = 4.32, see Sect. 7) is fitted with the model of Schreiber et al. (2018). The SED of AGS17 (bottom panel), which has no known redshift, is simply presented with an interpolation between the observed flux densities to illustrate that it peaks around 400 μm. This peak is inconsistent with the redshifts of the two optical sources with ID_CANDELS 4414 (z = 1.85) and 4436 (z = 0.92)