velength properties of the AzTEC/SMA sources is presented elsewhere (Yun et al., in prep.). In this paper, we examine the rest-frame optical/near-IR properties of a large sample of well-studied SMGs and the utility of the IRAC colour–colour plots for identifying and investigating the nature of the SMG population in general.

![image](figure1.eps){width="5.5in"}

[[*S*pitzer]{}]{} IRAC colours of Submillimetre Galaxies and AGNs {#sec:colourcolour}
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Red IRAC colours of SMGs
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A colour–colour diagram is a powerful tool for analyzing SEDs sampled coarsely with just a few broad-band measurements. The four [[*S*pitzer]{}]{} IRAC bands (3.6 , 4.5 , 5.8 , and 8.0 ) probe the portion of the galaxy SED that includes photospheric emission from cool stars (at $z=0-4$) and power-law continuum from hot dust surrounding young stars and/or AGN. Polycyclic aromatic hydrocarbon (PAH) and other spectral features are also important for the $z\sim0$ galaxies. The 1.6  stellar photospheric PAH feature is prominent in stellar systems older than 10 Myr, and galaxies with substantial cool stellar populations appear blue in an IRAC colour–colour diagram as a result [see @simpson99; @sawicki02]. Sources dominated by stellar photospheric emission form a densely concentrated cloud near ($-$0.4, $-$0.4) in the $S_{5.8}/S_{3.6}$ vs. $S_{8.0}/S_{4.5}$ colour–colour diagram for a sample of field galaxies shown by @lacy04. The $z\sim0$ late type galaxies with varying amounts of PAH emission in the 8.0  band form a distinct branch of galaxies with a constant $S_{5.8}/S_{3.6}$ ratio emerging from this cloud. Noting that 54 quasars identified in the Sloan Data Release 1 quasar survey [@schneider03] are associated with a second branch with red IR colours, Lacy et al. identified a wedge-shaped area in the colour–colour diagram as the characteristic region populated by obscured and unobscured AGNs.

To explore the importance of AGN activity among SMG