  We compare observed far-infrared (FIR) galaxy spectral energy distributions
(SEDs) at $z\sim6$ and theoretical predictions for first massive population II
(Pop II) galaxies. Observed FIR SEDs at $z\gtrsim5$ are broad and shifted to
bluer wavelengths when compared to galaxies at $z\sim3$. By implementing an
analytical model for dust emission from Pop II massive galaxies,we were able to
reproduce the observed behaviour as a consequence of the high energy densities
and silicate-rich dust composition of high-$z$ model galaxies. As noted in a
previous work, the non-blackbody nature of galaxy SEDs at $z \sim 6$ should be
taken into account when interpreting measurements of FIR luminosities to avoid
underestimating star formation rates.
