New insights on the distant AGN population
  Current X-ray surveys have proved to be essential tools in order to identify
and study AGNs across cosmic time. However, there is evidence that the most
heavily obscured AGNs are largely missing even in the deepest surveys. The
search for these obscured AGNs is one of the most outstanding issues of
extragalactic astronomy, since they are expected to make a major contribution
to the high energy peak of the X-ray background (XRB) and might constitute a
particularly active and dusty phase of black hole and galaxy evolution. Using a
newly developed SED fitting technique to decompose the AGN from the
star-formation emission in the infrared band we identify a sample of 17 IR
bright quasars at z=1-3. For the majority of these sources the X-ray spectrum
is well characterised by an absorbed power law model, revealing that ~25% of
the sources are unabsorbed; the remainder of these IR quasars have
moderate-to-high absorption (N_H>10^23 cm^-2), and ~25% are not detected in the
X-rays and are likely to be Compton thick (N_H>10^24 cm^-2). We therefore find
a much higher fraction of obscured than unobscured quasars, indicating that a
large fraction of the luminous black hole accretion was very heavily obscured
at z~2.
