Spitzer/IRAC [5.8]–[8.0] versus [3.6]–[4.5] colour–colour diagram for SMGs and IR QSOs in AB magnitudes. (a) All symbols and models plotted are identical to those in Fig. 1. The thin long-dashed line outlined the AGN region proposed by Stern et al. (2005). The solid line shows the extended region we propose for the identification of the SMG counterparts (equation 2). (a) The same Spitzer/IRAC colour–colour diagram for IRAC sources found within a 6 arcsec radius of the nine securely identified mm/submm sources (filled circle). Empty circles are foreground/interloper IRAC sources found within the same search radius as discussed in the text. The thick-solid line represents power-law spectrum sources with α= 0.3– 1.0 (S_ν∝ν^−α) in both panels. (c) and (d) The same theoretical colour–colour evolution tracks by Efstathiou et al. (2000) shown in Fig. 1 are plotted for comparison.