2008A&A...484..671G

The efficiency of mid-infrared selection methods for finding obscured AGN is investigated using data in the Chandra Deep Field North. It is shown that samples of AGN candidates compiled on the basis of mid-infrared colours only suffer substantial contamination from normal galaxies. X-ray stacking analysis reveals a soft mean X-ray spectrum for these sources, consistent with Γ≃2.1. This suggests that star-forming galaxies and not obscured AGN dominate the stacked signal. In contrast AGN selection methods that combine mid-infrared with optical criteria are more successful in finding heavily obscured AGN candidates. A method similar to the one proposed by Fiore et al. (2008ApJ...672...94F) is adopted to select extremely red objects (R-[3.6]>3.7mag) with high 24µm to optical flux ratio (f_24µm/f_R>1000). About 80% of these sources are not detected at X-ray wavelengths. Stacking the X-ray photons at the positions of these sources shows a flat mean X-ray spectrum (Γ≃0.8), which suggests Compton-thick sources, low-luminosity and moderately obscured (N_H∼8x10²²cm^–2) AGN, or a combination of the two. The mid-infrared colours and luminosities of these sources are consistent with ULIRGs at z≃2, while HST/ACS images, available for the optically brighter of these sources, show disturbed optical morphologies in many of them. The evidence above suggests that this population includes systems in the process of formation.