2008ApJ...672...94F

We make use of deep HST, VLT, Spitzer, and Chandra data on the Chandra Deep Field-South to constrain the number of Compton-thick AGNs in this field. We show that sources with high 24 µm-to-optical flux ratios and red colors form a distinct source population, and that their infrared luminosity is dominated by AGN emission. Analysis of the X-ray properties of these extreme sources shows that most of them (80%±15%) are indeed likely to be highly obscured, Compton-thick AGNs. The number of infrared-selected, Compton-thick AGNs with 5.8 µm luminosity higher than 10^44.2 ergs/s turns out to be similar to that of X-ray-selected, unobscured, and moderately obscured AGNs with 2-10 keV luminosity higher than 10⁴³ ergs/s in the redshift bin 1.2-2.6. This ``factor of 2'' source population is exactly what is needed to solve the discrepancies between model predictions and X-ray AGN selection.