Addressing the Missing AGN Problem

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• Belinda Wilkes (SAO)
• Thursday 17 May 2018, 16:00-17:00
• Sackler Lecture Theatre, IoA (tea at 3:30 pm).

If you have a question about this talk, please contact ma557.

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.

This talk is included in these lists:

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• Institute of Astronomy Colloquia
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• Priscilla
• Sackler Lecture Theatre, IoA (tea at 3:30 pm)

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