1. The Evolution of AGN Obscuration
Ezequiel Treister (ESO)
Meg Urry (Yale)
Julian Krolik (JHU)
Shanil Virani (Yale)
Priya Natarajan (Yale)

2. Active Galactic Nucleus (AGN)
supermassive black hole
actively accreting matter
x galaxy luminosity from few lt-hrs
Host Galaxy
AGN are explained by accretion of matter into a super-massive black holes.
Masses of the black hole are 10^6-10^9 and the luminosity of the central parsec can be larger than the whole galaxy

3. The AGN Unified Model blazars, Type 1 Sy/QSO broad lines
AGN unification paradigm can explain the observed differences between type 1 and type 2 AGN.
Urry & Padovani, 1995

4. The AGN Unified Model radio galaxies, Type 2 Sy/QSO narrow lines
Urry & Padovani, 1995

5. Supermassive Black Holes
Many obscured by gas and dust
How do we know that?
Local AGN Unification
Explain Extragalactic X-ray “Background”
How do we know that there are many obscured BHs?
Locally the obscured/unobscured ratio is ~3:1
More importantly, this is the case at all redshifts because of the X-ray background
Credit: ESO/NASA, the AVO project and Paolo Padovani

6. Observed X-ray “Background”
We know now that the X-ray background is just collective emission from previously unresolved sources,
90% of them AGN.
The X-ray background spectrum is very steep, with a peak at ~30 keV.
This is good indication that obscured AGN should outnumber the unobscured ones.
Frontera et al. (2006)

7. AGN in X-rays Increasing NH Photoelectric absorption
affect mostly low energy
emission making the
observed spectrum
look harder.
The reason is that obscuration makes the X-ray spectrum appear harder. In these units, an unobscured AGN
is roughly flat, while as we add more and more obscuration (parameterized as a neutral hydrogen column density)
the observed spectrum appears harder and harder, thus closer to the observed X-ray background.
Increasing NH

8. X-ray Background
XRB well explained using a combination of obscured and unobscured AGN.
# w NH > 1023 cm-2
still uncertain.
With this idea in mind, for almost 20 years now people tried to use a combination of obscured
and unobscured AGN to fit the spectral shape and intensity of the XRB.
The most important parameter is of course the assumed fraction of obscured AGN.
Here I show an example for our later work, in this case assuming a fraction of obscured to unobscured AGN of ~3:1.
Setti & Woltjer 1989
Madau et al. 1994
Comastri et al. 1995
Gilli et al. 1999,2001
And others…
Treister & Urry, 2005

9. What do we know so far? Problems:
More obscured AGN at low luminosity (Steffen et al. 2003, Ueda et al. 2003, Barger et al. 2005, Akylas et al. 2006)
More obscured AGN at high-z? (Ueda et al. 2003: No, La Franca et al. 2005: yes, Ballantyne et al. 2006: yes)
Problems:
Low number of sources
Selection effects:
- X-ray selection (missed obscured sources)
- Optical incompleteness (no redshifts)
- X-ray classification: “K correction”
But, what do we really know about the fraction of obscured AGN?
Thanks to X-ray surveys, it is now well established that there should be relatively more obscured AGN at lower luminosities
It is possible that there is also evolution in the fraction of obscured AGN, however even X-ray surveys have several problems:
Because of these problems, we decided to generate the largest possible sample of X-ray selected AGN in order to
study the fraction of obscured AGN.

10. Meta-Survey 7 Surveys, 2341 AGN, 1229 w Ids 631 Obscured
(no broad lines)
1042<Lx<1046, 0<z<5
This sample is ~4X larger than previous studies and includes both large area/shallow sources
in order to properly sample the high luminosity sources and deep surveys for the low luminosity/high redshift AGN.
This is the X-ray sensitivity function, which only tells you part of the story, because since redshifts are required
to calculate luminosities and identify sources. Hence, the optical flux should be considered as well.
Treister & Urry, 2006

11. Total effective area of meta-survey
This is our sensitivity plot that we will use to correct for the effects of incompleteness. As you can see
the area covered is now a function of both the X-ray flux and the optical magnitude.
Treister & Urry, 2006

12. Ratio vs Redshift Treister & Urry, 2006
First, we studied the dependence of the fraction of obscured AGN on redshift. Our full sample
has a completeness of ~50%. Data points show the “raw” data, not corrected for any selection
effects, while the solid lines shows the expectation for a intrinsically-constant fraction if incompleteness
is taken into account.
Treister & Urry, 2006

13. Ratio vs Redshift Treister & Urry, 2006
Same plot but increasing the completeness by restricting our sample. As you
could expect, the observed fraction gets flatter with higher completeness. Same
with the expected curve.
Treister & Urry, 2006

14. Ratio vs Redshift Treister & Urry, 2006
This is now for a very high completeness of ~90%. Here it starts to appear
like the observed fraction actually increases with redshift.
Treister & Urry, 2006

15. Ratio vs Redshift Key input: Luminosity
dependence of obscured AGN fraction.
In order to correct for the effects of incompleteness, we divide the observed data
points by the expectation for a constant fraction, so that the bottom panel shows
the fraction of obscured AGN relative to a constant value.
3 reasons for decline:
Lose absorbed X-ray sources (K correction helps you)
Lose redshifts because of optical magnitude
most important effect: luminosity dependence.
See also:
La Franca et al. 2005
Ballantyne et al. 2006
Akylas et al. 2006
Treister & Urry, 2006

16. Ratio vs Luminosity Treister & Urry, 2006
Hence, it makes sense to study the dependence of the fraction of luminosity.
As you can see, a simple linear parameterization on log L does a very good job
explaining the observational results.
Treister & Urry, 2006

17. Ratio vs Luminosity Treister & Urry, 2006
For example, the latest X-ray background fitting of Gilli et al. which includes
a more complex parameterization fails to explain the observed data.
Treister & Urry, 2006

18. Ratio vs Luminosity Treister & Urry, 2006 Hasinger et al.
Even if we add recent data from a similar sample by Gunther Hasinger there is a clear discrepancy at high luminosities.
So, it is clear that there is a luminosity dependence of the obscured AGN fraction. Now, we would like to understand
why this is the case.
Hasinger et al.
Treister & Urry, 2006

19. The AGN Unified Model Urry & Padovani, 1995
The simplest unified model tells you that the near and mid IR emission comes from
the torus regions closer to the BH.
In this model, the relative fraction of IR to bolometric emission is a direct measure of
the fraction of sky covered by the torus, and hence of the fraction of obscured AGN.
Urry & Padovani, 1995

20. A Changing Torus? Granato & Danese Torus model ~2x
A change in the IR/NUV
flux ratio may indicate a
change in torus geometry.
Granato & Danese
Torus model
~2x
If the change in the fraction of obscured AGN is due to a change in the opening
angle, then most IR torus models (in this case from the model of Granato & Danese)
predict that the IR emission is scaled proportionally.

21. Torus Structure Sample
Completely unobscured AGN
Narrow redshift range, 0.8<z<1.2
Wide range in luminosity
Data at 24 µm from Spitzer
High L
SDSS DR5 Quasar sample
11938 quasars, 0.8<z<1.2
192 with Spitzer 24 µm photometry
157 of them with GALEX UV data
Low L
GOODS: North+South fields
10 unobscured AGN
All Spitzer 24 µm photometry
8 with GALEX UV data
In order to study this we constructed a large sample of unobscured AGN in
a narrow redshift range in order to look for a dependence of the fraction of IR
emission on luminosity.
Mid L
COSMOS
19 unobscured AGN
14 Spitzer 24 µm photometry
All with GALEX UV data

22. Torus Structure Bolometric luminosity constructed from NUV to mid-IR.
No change in NUV/Bol
ratio with luminosity!
The first result is that at least the fraction of light at NUV wavelengths, coming directly from
the accretion disk does not depend on luminosity.
Treister et al, ApJ submitted.

23. Torus Structure Change in 24 µm/Bol ratio with luminosity!
Lower ratio at high L
Consistent with
larger opening angles
at higher luminosities.
However, we found a clear change, by a factor of ~3x in the fraction of IR light measured
at rest-frame 12 microns as a function of luminosity. This change goes in the right direction, namely
low luminosity AGN have a higher fraction of IR to bolometric luminosity, which can be interpreted
as a smaller opening angle.
Now, using torus models we can convert this fraction of IR light into a obscured fraction and
compare with the results from X-ray surveys.
Treister et al, ApJ submitted.

24. Fraction of Obscured AGN
Similar luminosity
dependence as found
on X-ray surveys.
Higher values from
fIR/fbol method.
Obscured AGN
missed by X-ray
surveys. Detected
in mid-IR?
This is our main result from this study.
In general there is a good agreement between the obscured AGN fraction from
X-ray surveys and from our method.
However, our results are systematically higher. Can this further indicate that there is
a large fraction of obscured AGN missed by X-ray surveys? Can these sources be detected
in the mid-IR?
Treister et al, ApJ submitted.

25. HST ACS color image (0.3% of GOODS)
This is a small fraction of the GOODS field, a deep multiwavelength survey with Chandra, Spitzer and HST
Showing three examples of X-ray sources, one faint in the optical and another completely invisible.

26. HST+Spitzer color image (0.3% of GOODS)
However, all these sources are bright in the IR, as expected since most of the
absorbed energy is re-emitted at these wavelengths.

27. IR Fraction vs Flux AGN are bright IR sources! Treister et al 2006
Indeed, by studying the fraction of sources classified as AGN in X-rays we found that this
fraction depends strongly on the IR flux, going from ~50% to 5%. This results holds
even correcting for the AGN missed in X-rays.
Treister et al 2006

28. IR Luminosity Distribution
On average, AGN are ~10x brighter than normal galaxies
For fainter AGN, the host galaxy makes a significant contribution
A similar result is found while studying the IR luminosity of X-ray selected AGN.
However, by computing the luminosity distribution for a random sample of normal
Galaxies with the same redshift distribution we found significant overlap. This indicates
that in particular for the fainter AGN the host galaxy makes a significant contribution to
the IR light.
Treister et al 2006

29. Infrared Background
AGN (+ host galaxy) contribute ~3-10% of the total extragalactic background light
Even though AGN are bright IR sources, they only contribute ~3-10% of
the IR background depending on wavelength. So, in contrast to the
X-ray background, AGN are a small fraction of the total extragalactic IR light.
Treister et al 2006

30. Compton Thick AGN Defined as obscured sources with NH>1024 cm-2.
Very hard to find (even in X-rays).
Observed locally and needed to explain the X-ray background.
Number density highly uncertain.
High energy (E>10 keV) observations are required to find them.
There is a population that was completely missed in all these studies.
Those are the most obscured sources. So obscured that are not detected
even in X-rays. Those are the CT AGN.
Even though we don’t know the exact density of these sources, they are
required in large numbers by XRB synthesis models and are observed locally.

31. INTEGRAL Survey PIs: Meg Urry, Shanil Virani, Ezequiel Treister
Exp. Time (Msec): 0.7 (archive)+1.5 (2005)+ 1 (2008). Deepest extragalactic INTEGRAL survey
Field: XMM-LSS (largest XMM field)
Flux limit: ~4x10-12 ergs cm-2 s-1 (20-40 keV)
Area: ~1,000 deg2
Sources: ~20
Obscured AGN: ~15 (~5 Compton-thick)
In order to study directly this population, at least at low redshifts,
three years ago we started a deep survey at high energies with INTEGRAL.

32. INTEGRAL Mosaic (2.2 Ms) Significance Image, 20-50 keV MCG-02-08-014
These are the first results. We found a total of 10 sources, including the “famous” CT
AGN NGC1068.
But, we recently got a nice surprise when we got the first 2 Mseconds of data. We
detected in INTEGRAL a source not detected in ROSAT, MCG
Significance Image, keV

33. MCG-02-08-014 z=0.0168 Optical class: Galaxy IR source (IRAS)
Radio source (NVSS)
Narrow line AGN
(based on OIII emission)
No soft X-rays (ROSAT)
Good CT AGN candidate
This source was only classified spectroscopically as an AGN
by the SDSS because narrow OIII was detected. It is also a
radio source, but not detected in soft X-rays, hence is a very good CT AGN candidate
in the nearby Universe.

34. INTEGRAL AGN logN-logS
In order to compare our results with the expectations from XRB models,
The first tool for that is the logN-logS plot. Here we show it including the results
from large area surveys (almost all-sky) at these energies. In general there is a good agreement.
Data points from
Beckmann et al. 2006
Treister et al, submitted

35. Space Density of CT AGN
X-ray background does not constrain density of CT AGN
We can also construct the logN-logS for CT AGN only. Here we found that
the CT AGN fraction is ~4x lower than what expected by models. This is because
the XRB does not constrain the density of CT AGN.
Treister et al, submitted

36. CT AGN and the XRB XRB Intensity Treister & Urry, 2005 HEAO-1 +40%
Indeed, for a given assumed XRB intensity, the CT AGN density is degenerated
with the normalization of the Compton reflection component, which contributes most
of the X-ray emission on CT AGN.
Treister et al, submitted

37. CT AGN and the XRB XRB Intensity HEAO-1 Original Treister & Urry, 2005
Gilli et al. 2007
Treister & Urry, 2005
Different models assumed different normalizations for the XRB, but now thanks to
the recent INTEGRAL and Swift results, it is clear that the original HEAO-1 intensity,
with a 10% uncertainty was correct, as assumed by the models of Gilli et al published
this year.
Treister et al, submitted

38. CT AGN and the XRB XRB Intensity HEAO-1 Original Treister & Urry, 2005
Gilli et al. 2007
Treister & Urry, 2005
Most likely solution
However, now we can constrain the density of CT AGN directly from the
INTEGRAL observations, finding that the most likely solution has a
CT AGN fraction ~4x lower than previously expected.
CT AGN
Space Density
Treister et al, submitted

39. X-ray Background Synthesis
So, we can now construct XRB population synthesis models
completely constrained by observations, finding that most
of the emission comes from sources with relatively low luminosities, 10^43-44 which
corresponds to bright seyferts or faint quasars.
Treister & Urry 2005

40. X-ray Background Synthesis
Similarly, we found that most of the emission comes from lower redshift
sources than previously expected, at z<1. This is a new result, first established
using X-ray surveys.
Treister & Urry 2005

41. Summary
AGN unification can account well for the observed properties of the X-ray background.
The obscured AGN fraction decreases with increasing luminosity.
Ratio of IR to Bolometric luminosity in unobscured AGN suggest this is due to a change in opening angle.
The obscured AGN fraction increases with redshift as (1+z)0.4.
AGN are luminous infrared sources, but contribute ~5% to extragalactic infrared background
Survey at high energies starts to constrain the spatial density of CT AGN.
Most important conclusions:
Fraction of obscured AGN depends on both luminosity and redshift
Luminosity dependence due to a change in opening angle
AGN are luminous IR sources but contribute only ~5% of the IR background.
We are starting to constrain the density of CT AGN using observations at high energies.