  We will briefly examine the following three issues related to the AGN
synthesis models for X-ray background: 1) the possibility that absorbed AGNs
evolve faster than unabsorbed ones; 2) the existence of the still debated
population of luminous absorbed AGNs, the so-called QSO2s; 3) the behavior of
the AGN density at a redshift above ~3.

Astrophysics