The Quest for a Complete Census of AGN Activity
  Deep X-ray surveys provide the most efficient identification of Active Galactic Nuclei (AGN) activity. However, recent evidence has indicated that the current <10 keV surveys do not detect the most heavily obscured AGNs. Here we explore whether the X-ray undetected AGN population can be identified within the ultra-deep Spitzer survey of the GOODS-N field using X-ray stacking techniques. We find evidence for AGN activity in the Spitzer dataset and the strongest and hardest X-ray signal is produced by galaxies with starburst-like infrared spectral slopes and median properties of z~0.8 and L_IR~10^{11} L_solar. The stacked X-ray properties suggest that obscured AGN activity is present in these sources, with a median X-ray spectral slope of Gamma~1 and L_X~10^{42} erg/s. These overall properties are consistent with the obscured AGN population expected to produce the unresolved X-ray background. 
D.M. Alexander (Durham)