  Submillimetre (sub-mm) galaxies have very high infrared (IR) luminosities and
are thousands of times more numerous at z~2 than local ultra-luminous IR
galaxies. They therefore represent a key phase in galaxy evolution which can be
missed in optical surveys. Determining their contribution to the global star
formation rate requires dissecting their IR emission into contributions from
starbursts (SB) and active galactic nuclei (AGN). There are several examples of
AGN systems which masquerade as SBs in either the IR or X-ray, and SBs can
often look like AGN in some wavebands. A combination of SB and AGN emission is
not unreasonable, given models of merger-driven evolution. To assess in detail
what powers the intense IR luminosity of sub-mm galaxies it is important to
obtain a complete multi-wavelength picture. Mid-IR spectroscopy is a
particularly good probe of where the intense IR luminosity is coming from. We
present the first results from a program to obtain Spitzer IRS spectroscopy of
a sample of high redshift galaxies in the GOODS-N field, a large fraction of
which are sub-mm galaxies. This field is already home to the deepest X-ray,
optical, IR and radio data. We piece together the sub-mm data with the Spitzer
photometry and IRS spectra to provide a well sampled IR spectral energy
distribution (SED) of sub-mm galaxies and determine the contribution to the
bolometric luminosity from the AGN and SB components.
