The Obscured X-ray Background and Evolution of AGN

Abstract

The X-ray Background has been resolved in the 0.5–$5{\rm\thinspace keV}$ band and found to consist mostly of both unabsorbed and absorbed AGN with column densities $<10^{23}\rm\thinspace cm^{-2}$. This contrasts with the local AGN population where the column density range extends to Compton-thick objects and beyond ($>1.5\times 10^{24}\rm\thinspace cm^{-2}$). Stacking analysis of the integrated emission of sources detected by XMM-Newton in the Lockman Hole, and by Chandra in the CDF-N and S reveals that the resolved fraction of the X-ray Background drops above $6{\rm\thinspace keV}$ and is about 50% above $8{\rm\thinspace keV}$. The missing flux has the spectrum of highly absorbed AGN, making it likely that the range of column density at redshift one is similar to that locally, and that many AGN are as yet undetected in well-studied fields.