3 What should observers take away from this?
See the video explanation, or read below
Buchner+15 worked out the
intrinsic number of AGN as a function of accretion luminosity, obscuring
screen density and redshift. This is possible in X-ray because the
selection function is well understood and independent of the host
Important results are:
- 3/4 AGN in the Universe are seen obscured. 3/4 of sight lines from
the central SMBH are obscured. More technically: The fraction of obscured
) averaged over cosmic time is
- 1/3 AGN in the Universe are seen Compton-thick. 1/3 of sight lines
from the central SMBH are Compton-thick. More technically: The fraction
of AGN with
averaged over cosmic time
, supported by ultra-hard
X-ray selection finding Ricci in prep., local infrared selection
by Annuar+14 finding , and another X-ray survey study by
Aird+15 finding ).
- The fraction of Compton-thick AGN does not seem to change with luminosity.
- The fraction of obscured AGN decreases with increasing luminosity.
, when not considering the
constant Compton-thick population. See figure, z=0.5-1:
- It is not generally true that the fraction of obscured AGN increases
with redshift. It increases for
- The luminosity-dependence has a critical turn-over luminosity
where it shows a peak (Burlon+11 in the local Universe, Buchner+15)
- The entire shape shifts to higher luminosities with redshift! (Buchner+15).
See figure. The star is always at the same position. Compare between
- Aim for opening angles of only
or a covering
fraction of . Everything else is
- For Compton-thick densities, aim for opening angles of
or a covering fraction of . (see illustration)
- Not all torii must look the same. But if you model lower opening angles,
keep in mind that another part of the population must have higher
obscuration. Think about them too.
- The obscuration-luminosity anti-correlation means that the obscuration
is tightly linked to the accretion process, and therefore nuclear
and should be explained by torus models. Try to make a model that
increases the covering with luminosities, and then decreases it again
with high luminosities. This could be explicitly through a causation
(feeding), or implicitly due to a correlation.
- Redshift-evolution: If you have a critical luminosity
where this effect sets on,
has to depend on something
else that could evolve with redshift. For example, if it is dependent
on Eddington ratio, the mass can evolve over redshift (downsizing
in black hole mass).
- Note that just putting more gas at high redshift is ruled out: This
at all luminosities (relation goes
up), not shift the peak luminosity.
- Buchner+15 systematically discusses a few broad classes of models
and why they are ruled out.
- If you select AGN at some specific redshifts, and compare to a lower
redshift bin at the same luminosity:
- You are not looking at the same population. Scale down the luminosities
you compare to.
- When you create an obscured AGN sample and an unobscured AGN sample:
- The obscured sample will have a lower average luminosity, therefore
either lower Eddington rate or higher mass.
- Use intrinsic luminosity matching.
(Web|ADS|PDF) Buchner et al. (2014) -- We investigated the opening angle and geometry of the torus through a new Bayesian Spectral analysis method applied to Chandra Deep Field South data.
(Web|ADS|PDF) Buchner et al. (2015) -- With a large sample spreading X-ray luminosity, redshift and column density, we plot
these quantities against each other (incorporating selection effects). More technically, the space density as a function of L,z,NH is
investigated, as well as the fraction of Compton-thick AGN, the fraction of obscured AGN. We also discuss torus models that could reproduce
the luminosity-dependent obscuration and its evolution.
(PDF|image) Poster for the TORUS2015 conference. Contains a comparison to the model of Wada (2012), which
roughly reproduces the expected column density distribution.
AGN catalogues for the CDFS and AEGIS-X fields are released at the MPE X-ray surveys website. This includes detection and redshift catalogues as well as derived spectral parameters, such as luminosities, column densities and probability of the object being obscured/Compton-thick.
Contact me for questions, any additional plots or the computed space densities. The space densities are available as a table in txt and fits format, with the columns described in the CDS catalogue.