Colour–colour diagram of IRAC sources. Green solid circles indicate sources in region A; red open dots show sources in region B; and blue solid triangles represent sources in region C. The cyan full squares denote sources at the Bullet’s redshift (see Sect. 6). The black contour levels represent C–COSMOS sources. The black solid square and diamond represent two galaxy templates, M 82 and NGC 4429, respectively, which were moved at the redshift of the Bullet. The black dashed line indicates the empirically determined region, largely populated by luminous unobscured AGN (Stern et al. 2005).