Upper left: colour map obtained directly from the calibrated 4.5 and 3.6 μm images. Upper right: colour map obtained from the colours of stars identified via PSF fitting in the 4.5 and 3.6 μm images. An average intrinsic colour of [3.6 − 4.5] = −0.15 mag (see Sect. 2.4) was subtracted from both maps. Lower left: median-smoothed 3.6 μm image. Lower right: difference of left upper map minus right upper map.