  High-redshift submillimetre-bright galaxies identified by blank field surveys
at millimetre and submillimetre wavelengths appear in the region of the IRAC
colour-colour diagrams previously identified as the domain of luminous active
galactic nuclei (AGNs). Our analysis using a set of empirical and theoretical
dusty starburst spectral energy distribution (SED) models shows that power-law
continuum sources associated with hot dust heated by young (<100 Myr old),
extreme starbursts at z>2 also occupy the same general area as AGNs in the IRAC
colour-colour plots. A detailed comparison of the IRAC colours and SEDs
demonstrates that the two populations are distinct from each other, with
submillimetre-bright galaxies having a systematically flatter IRAC spectrum (>1
mag bluer in the observed [4.5]-[8.0] colour). Only about 20% of the objects
overlap in the colour-colour plots, and this low fraction suggests that
submillimetre galaxies powered by a dust-obscured AGN are not common. The red
IR colours of the submillimetre galaxies are distinct from those of the
ubiquitous foreground IRAC sources, and we propose a set of IR colour selection
criteria for identifying SMG counterparts that can be used even in the absence
of radio or Spitzer MIPS 24 micron data.
