3.6   μmvs. 24   μm emission. Dots represent the overall 3.6   μm population with a 24μm counterpart, crosses the counterparts of X-ray sources, black filled circles the sources of Table 1 detected with the HST and red open circles the sources of Table 1 not detected with the HST. The lines mark log (f_3.6μm/f_24μm) = 0, − 1, − 1.5. (This figure is available in color in electronic form.)