  The spectral energy distributions (SEDs) of dusty high-redshift galaxies are
poorly sampled in frequency and spatially unresolved. Their form is crucially
important for estimating the large luminosities of these galaxies accurately,
for providing circumstantial evidence concerning their power sources, and for
estimating their redshifts in the absence of spectroscopic information. We
discuss the suite of parameters necessary to describe their SEDs adequately
without introducing unnecessary complexity. We compare directly four popular
descriptions, explain the key degeneracies between the parameters in each when
confronted with data, and highlight the differences in their best-fitting
values. Using one representative SED model, we show that fitting to even a
large number of radio, submillimetre and far-infrared (far-IR) continuum
colours provides almost no power to discriminate between the redshift and dust
temperature of an observed galaxy, unless an accurate relationship with a tight
scatter exists between luminosity and temperature for the whole galaxy
population. We review our knowledge of this luminosity-dust temperature
relation derived from three galaxy samples, to better understand the size of
these uncertainties. Contrary to recent claims, we stress that far-IR-based
photometric redshifts are unlikely to be sufficiently accurate to impose useful
constraints on models of galaxy evolution: finding spectroscopic redshifts for
distant dusty galaxies will remain essential.
