Astronomy Colloquium - Belinda Wilkes
- Belinda Wilkes
It is well-established that obscuration of the nuclear regions of active
galactic nuclei (AGN) are anisotropic. The effects on the observed emission
are strongly frequency dependent and lead to complex selection effects for
observations in most wavebands, including the X-ray. The result of these
selections effects is the lack of identification and/or detection of obscured
AGN in most surveys over much of the parameter space they populate.
Thus their numbers are under-estimated, particularly in flux-limited samples,
and observed trends are difficult to disentangle from the effects of selection,
with which they generally align. Low-frequency radio-selection offers an
alternative, albeit for those AGN that are radio-bright. Multi-wavelength
studies of the low-frequency radio-selected, orientation unbiased, 3CR sample
of luminous AGN recognize highly obscured AGN, and can separate intrinsic
physical from observed, orientation-dependent properties.
Chandra, Spitzer, Herschel and multi-wavelength observations show that
~half the redshift, z=0.5-2 3CR sub-sample is significantly obscured
with ~a quarter being Compton thick, a larger fraction than estimated
for optically- and X-ray-selected high-luminosity samples.
I will discuss the challenges of detecting, and when detected,
estimating obscuration levels, for sources approaching the flux limit of
X-ray surveys where the observed spectral slope or hardness does not
provide a reliable estimate of the obscuration. As a result, many
obscured AGN are either lost from X-ray surveys, or incorrectly
classified and their intrinsic luminosities underestimated by factors of
10-1000. This may explain the discrepant obscured fractions reported for
optical- and X-ray-samples with no independent measure of the AGN luminosity.
I will close with a look to the future, including ongoing LOFAR radio surveys,
soon-to-be-launched e-ROSITA’s all sky X-ray survey, and the Lynx Mission Study,
looking towards a Chandra successor in the future.