Spitzer/IRAC colour-colour diagram of the GOODS-N sample and our low-redshift sample (red dots). All of our sample galaxies are not MIR power-law AGN. The large error bars in the [5.8]−[8.0] colour is caused by the low sensitivity of the 5.8 μm image data. The low [3.6]−[4.5] colour indicates that our low-mass galaxy sample contains very few AGNs. The black dots are the galaxies in CANDELS GOODS-North field as a comparison.