  We present SEDs, Spitzer colours, and IR luminosities for 850 micron selected
galaxies in the GOODS-N field. Using the deep Spitzer Legacy images and new
data and reductions of the VLA-HDF radio data, we find statistically secure
counterparts for 60 per cent (21/35) of our submm sample, and identify
tentative counterparts for another 12 objects. This is the largest sample of
submm galaxies with statistically secure counterparts detected in the radio and
with Spitzer. We find that in most cases the 850 micron emission is dominated
by a single 24 micron source. A composite rest-frame SED shows that the submm
sources peak at longer wavelengths than those of local ULIRGs of the same
luminosity and therefore appear to be cooler. The SEDs of submm galaxies are
also different from those of their high redshift neighbours, the near-IR
selected BzK galaxies, whose mid-IR to radio SEDs are more like those of local
ULIRGs. Using 24 micron. 850 micron and 1.4 GHz observations, we fit templates
that span the mid-IR through radio to derived the integrated IR luminosity of
the submm galaxies. By themselves, 24 micron and radio fluxes are able to
predict LIR reasonably well because they are relatively insensitive to
temperature. However, the submm flux by itself consistently overpredicts LIR
when using spectral templates which obey the local ULIRG temperature-luminosity
relation. The shorter Spitzer wavelengths sample the stellar bump at the
redshifts of the submm sources, and we find that the Spitzer photometry alone
provides a model independent estimate of the redshift. Using X-ray and mid-IR
data, only 5 per cent of our secure counterparts show strong evidence for an
active galactic nucleus dominating the IR luminosity.
