Mid-infrared (24 μm) to optical (R_C) flux ratio versus R_C − K colour of the X-ray AGN in the Lockman Hole. The blue and red symbols are identical with those in Fig. 4, while the grey dots represent all optical-IRAC-MIPS sources in the Lockman Hole. The black lines are the tracks of QSO and Arp 220 SEDs with 0 < z < 3, while the violet lines are tracks of SED combinations between QSOs and host galaxies (non type-1 Seyferts). Green lines from right to left are tracks of elliptical, spirals, and irregular galaxies. This figure is available in color in electronic form.