Too few Compton-thick AGNs to explain the Cosmic X-ray background
INTEGRAL studies by L. Bassani et al. (ApJ 636, L65) and V. Beckmann
et al. (astro-ph/0606687) conclude that the fraction of Compton-thick
AGN is less than 10% and insufficient to account for the CXB emission,
at least in the local universe. This is consistent with the results
from the first census of the sky with IBIS above 100 keV by A. Bazzano
et al. (ApJ accepted; astro-ph/0608164), which indicates that while
Galactic emission in the 100-300 keV range is strongly dominated by
point sources, active galaxies detected above 1 mCrab account for only
~3% of the cosmic hard X-ray background.
Surveys at lower energies demonstrated that in the classical X-ray
band the CXB can be largely explained by AGN with an increasing fraction
of absorbed sources. Extrapolating from this, one would have expected
a much larger contribution from heavily shrouded AGNs than now found by
the different INTEGRAL studies. Possibly shrouded sources are more
frequent at higher redshifts where they would be too weak to detect
with the current sensitivity limits. Or another population of even
more densely shrouded sources could exist.
A larger sample of AGNs was studied by Sazonov et al. (astro-ph/0608418)
basically confirming the results described above.
The picture shows an artist impression of a dust torus surrounding a
supermassive black hole and an accretion disk. In this case the thick
torus will block the optical light emitted from the center, but hard
X-rays and gamma-rays will mostly be able to pass through the torus.
