Institute of Astronomy

Addressing the Missing AGN Problem

SpeakerTalk DateTalk Series
Belinda Wilkes (SAO)17 May 2018Institute of Astronomy Colloquia


The complex nuclear structures of active galactic nuclei (AGN)
lead to strong selection effects in most wavebands, including the X-ray.
Highly obscured AGN are hard to find, and identify. Estimating their
numbers, a function of luminosity and redshift, remains a major quest
both for AGN science, and in understanding the level of accretion
power particularly in the early Universe.

== Multi-wavelength observations of the low-frequency, radio-selected 3CR
AGN sample (z>0.5) largely avoid selection biases, revealing the obscured
and probing both their intrinsic, and orientation-dependent properties.
Chandra, Spitzer, Herschel and multi-wavelength observations confirm that
the FIR (> ~ 40um) does not depend on orientation and that ~half the sample
is significantly obscured with ~a quarter being Compton thick. This is a
larger fraction than typically estimated for optically- or X-ray-selected,
high-luminosity samples. Once the primary X-ray power-law is obscured, AGN
X-ray spectra are complex, and detecting and estimating X-ray obscuration
levels becomes highly uncertain. This is particularly true for sources close
to the flux limit. The loss or miss-classification of obscured AGN in
also results in large (*10-1000) uncertainties on their intrinsic
This may explain discrepant obscured fractions reported for various
optical- and X-ray-samples, and may also affect the shape of derived
functions. The use of independent measures of the AGN power, such
as the low-frequency radio, or [OIII] emission line luminosity, helps to
counteract such problems. ==

I will close with a look to the future, reviewing the science pillars
and mission concept of the NASA-funded, Chandra Successor Mission Study,
leading up to the USA Astronomy and Astrophysics Decadel Survey.


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