The Evolution of Obscuration in AGN

Details The Evolution of Obscuration in AGN The Evolution of Obscuration in AGN

Sep 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006head....9.1007t&link_type=abstract

American Astronomical Society, HEAD meeting #9, #10.07; Bulletin of the American Astronomical Society, Vol. 38, p.364

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Scientific paper

One fundamental ingredient in our understanding of the AGN population
is the ratio of obscured to unobscured AGN and whether this ratio
depends on other parameters like intrinsic luminosity or
redshift. Observationally, deep X-ray surveys found that the obscured
AGN fraction depends on luminosity. However, the dependence on
redshift is less clear. In this work, we constructed the largest
sample to date of AGN selected in hard X-rays, containing a total of
1229 sources, 631 of them obscured, with a high spectroscopic
completeness in order to study the possible dependence of the fraction
of obscured sources with redshift and/or luminosity. We confirm that
this fraction decreases with increasing luminosity as previously
reported and found that at the same time it increases with increasing
redshift. This is the first time that this evolution is significantly
detected using only optical spectroscopy to separate obscured and
unobscured AGN. Additionally, we use the spectral shape and intensity
of the X-ray background as a separate constraint on the evolution of
the obscured AGN fraction finding consistent results. This result can
be interpreted as an evolution in the location of the obscuration,
from the central parsec-scale region (the torus) at low redshift to
kiloparsec scales (the host galaxy) at high redshift, as it is known
that most galaxies contained more dust in the past.
Using these results, we calculate the integrated bolometric AGN
emission finding it to be at most 5% of the total extragalactic
light. Hence, while AGN contribute most of the light at X-ray
wavelengths, they constitute only a small fraction of the integrated
extragalactic light.
We thank the support of the Centro de Astrof\'{\i}sica FONDAP and from
NASA/{\it INTEGRAL} grant NNG05GM79G.

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