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Addressing the Missing AGN Problem

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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 luminous AGN sample (z>0.5) largely avoid selection biases, revealing the obscured AGN, 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 surveys also results in large (*10-1000) uncertainties on their intrinsic luminosities. This may explain discrepant obscured fractions reported for various optical- and X-ray-samples, and may also affect the shape of derived luminosity 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, Lynx, leading up to the USA Astronomy and Astrophysics Decadel Survey.

This talk is part of the Institute of Astronomy Colloquia series.

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