By comparing an optically selected sample of narrow lines AGN with an X-ray
selected sample of AGN we have recently derived an estimate of the intrinsic
(i.e. before absorption) 2-10 keV luminosity function (XLF) of Compton Thick
AGNs. We will use this XLF to derive the number of Compton Thick AGN that will
be found in the SIMBOL-X survey(s).Comment: Talk at the Simbol-X symposium held in Paris, 2-5 December, 2008. 6
  pages, 1 figure with three panel

A. Caccianiga

Jéro^me Rodriguez

P. Severgnini

Phillippe Ferrando

R. Della Ceca

English

arXiv

Crossref

Heavily obscured AGN with SIMBOL-X
R. Della Ceca, A. Caccianiga and P. Severgnini
INAF-Osservatorio Astronomico di Brera, Milano
Abstract. By comparing an optically selected sample of narrow lines AGN with an X-ray selected
sample of AGN we have recently derived an estimate of the intrinsic (i.e. before absorption) 2-
10 keV luminosity function (XLF) of Compton Thick AGNs. We will use this XLF to derive the
number of Compton Thick AGN that will be found in the SIMBOL-X survey(s).
Keywords: X-ray; Active Galaxies; X-ray sources; Observational Cosmology
PACS: 95.85.Nv; 98.54.Cm; 98.70.Qy; 98.80.Es
INTRODUCTION
The discovery of quiescent Super-Massive Black-Holes (SMBH) in the nuclei of non-
active nearby galaxies with prominent bulges ([11], [12]), along with the discovery of
scaling relations between the central BH mass and galaxy properties (e.g. bulge lumi-
nosity/mass and velocity dispersion, [9], [6]) strongly suggest a tight link between BH
growth and galaxy evolution (commonly referred to as "co-evolution") and a physical
mechanism linking accretion and ejection of material occurring in the central (sub-
parsec scale) galaxy nucleus to the rest of the galaxy. This implies that AGN are not
only sources where high energy physical processes take place but are leading actors
in the formation and evolution of galaxies and, in general, of cosmic structures in the
Universe.
However the largest fraction of the AGN population is obscured by a large amount of
cold matter around the Active Nuclei that does not permit a direct view to the central
energy source. For absorbed AGN having column densities (NH) up to few times 1023
cm −2 the current missions at E<10 keV (XMM-Newton and Chandra) are producing
a wealth of useful data that can be used to evaluate their physical and cosmological
properties (e.g. see [3] and [5] and references therein). On the contrary, the situation
is almost completely unconstrained for the sources having NH greater than 1024 cm −2
(the so called Compton Thick AGN, where the matter is optically thick to Compton
scattering). According to the synthesis modeling of the cosmic X-ray background (XRB)
([17], [8]), these objects should represent ∼50% of the total absorbed AGN population,
although recent revisions ([18]) seem to suggest a smaller contribution.
In [4] we have reviewed the Compton Thick AGN studied so far (using the current
and past X-ray missions, i.e. ASCA, BeppoSAX, XMM-Newton, Chandra, INTEGRAL,
SWIFT and SUZAKU) and discussed the X-ray spectral improvements offered by the
Simbol-X mission (see also the contributions of W. Beckmann, F. Panessa and F. Fiore at
this meeting). We discuss here their expected cosmological properties (LogN-LogS and z
distribution). (Ho,ΩM,ΩΛ)=(65,0.3,0.7) are used in this paper. Finally we are considering
here only Compton Thick AGN with NH between 1024 and 1025 cm−2.
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THE XLF OF COMPTON-THICK AGN
In [3] by comparing the results obtained using an hard (4.5-7.5 keV) X-ray selected
sample of AGN with those derived by using an optically selected sample of narrow
lines AGN ([16]), we have evaluated, in an indirect way, the intrinsic (i.e. before the
absorption) XLF of Compton Thick AGN (at z=0) in the 2-10 keV energy range. This
XLF (thick solid line in figure 1, left panel) is described by a smoothly connected two
power-law function, dΦ(Lx,z=0)dLogLx = A[(
Lx
L?
)γ1 +(LxL? )
γ2 ]−1, with γ1 = 1.55, γ2 = 2.61, Log
L? = 44 and A = 4.79× 10−7 h365 Mpc
−3. A comparison with the results from other
projects is also reported in figure 1 (left panel). As can be clearly noted the different
estimates of the space density of Compton Thick AGN are consistent (roughly within a
factor 2, see the two dashed lines) with the proposed XLF of Compton Thick AGN, with
a possible hint of a XLF flattening below few times 1042 erg s−1; we emphasize that
this agreement is remarkably good if we consider that the different estimates have been
derived using very different selection criteria and methods. Finally the consideration
that the total number of Compton Thick AGN cannot be increased arbitrarily, without
violating the limits imposed by the local black mass density derived from dynamical
studies of local galaxy bulges[13], allows us to put some constraints on the space density
of optically/infrared-elusive Compton Thick AGN; this limit is violated if the total
density of Compton Thick AGN (including also the optically-elusive ones) is above
the upper dashed envelope reported in figure 1 (left panel, see [3] for details).
PREDICTIONS FOR THE SIMBOL-X MISSION
Having derived an estimate of the intrinsic XLF of Compton Thick AGN (at z=0) we
can now make predictions for the Compton Thick AGN population that will be detected
above 10 kev (in particular in the 10-40 keV energy range) with SIMBOL-X.
By integrating the XLF reported in figure 1 (LX from 1042 to 1048 erg s−1; z from 0
to 3) using the best fit cosmological evolution parameters reported in [3] and convolving
the results with a) the modeling of X-ray spectra transmitted through Compton-thick
absorbers as discussed in [14] and b) a flat NH distribution between 1024 and 1025 cm−2,
we predict the LogN(>S)-LogS reported as solid line in figure 1 (middle panel).
In this figure we also shown:
a) the surface density of Compton Thick AGN obtained from the SWIFT/BAT survey
([19]) 1. To this purpose we have used the 5 Compton Thick AGN (as revised in [18],
i.e. MKN3, NGC3281, NGC4945, NGC5728 and NGC7582) having NH between 1024
and 1025 cm −2 and F14−195keV > 6× 10−11 erg cm−2 s−1 (where we have a flat sky
coverage and a covered area of∼ 30526 deg−2). To convert the fluxes from the different
energy bands we have used an absorbed power law spectral model having photon index
Γ = 1.9 and NH ∼ 3×1024 cm −2 (F10−40keV ∼ 0.32×F14−195keV );
1 The SWIFT/BAT results are fully consistent with those obtained from the INTEGRAL surveys
(L.Bassani, private communication)
FIGURE 1. Left panel: The thick solid line represents the proposed intrinsic 2-10 keV XLF of Compton
Thick AGN (at z=0), while the two dashed lines correspond to an uncertainty of a factor two on the XLF
normalization. The filled circles at L(2−10keV ) ∼ 3× 1042 and ∼ 3× 1043 erg s−1 represent the density
of local Compton Thick AGN derived using the INTEGRAL all sky survey discussed in [15]. The filled
triangle represents the density of Compton Thick AGN with LX ∼ (1− 4)× 1043 erg s−1 as derived by
[2]. The filled squares represent the density of Compton Thick AGN with LX ∼ (3−10)×1043 erg s−1
and LX ∼ (1− 10)× 1045 erg s−1 , as derived by [7], while the open square at LX ∼ (1− 10)× 1045
erg s−1 corresponds to the density of Compton Thick AGN as derived by [1]. The densities of Thick AGN
reported in [2], [7] and [1] have been rescaled to z=0 using the best fit evolutionary model discussed in [3].
Middle panel: The solid line is the LogN(>S)-LogS in the 10-40 keV energy range derived as explained
in the text. We have also reported the expected flux limits of the All Sky EXIST mission (“1 year survey
data") and SIMBOL-X (1 Msec exposure). The filled dot at ∼ 2× 10−11 erg cm−2 s−1 represents the
surface density of Compton Thick AGN obtained from the SWIFT/BAT survey, the short dashed line is
the LogN(>S)-LogS of Compton Thick AGN from [18] while the long-dashed line are the prediction
from [8] based on the XRB modeling. Right panel: Expected redshift distribution for the Compton Thick
AGN population at the SIMBOL-X flux limit of F10−40keV ∼ 7×10−15 erg cm−2 s−1
b) the LogN(>S)-LogS of Compton Thick AGN (NH between 1024 and 1025 cm−2)
as derived by [18] (short-dashed line) using SWIFT/BAT and INTEGRAL data and
c) the predictions based on the XRB modeling (long-dashed line. R. Gilli, private
communication).
As can be clearly seen we find a very good agreement between our predictions and
the measurements performed so far with SWIFT/BAT, making us confident that our
predictions for the SIMBOL-X mission are robust. We also find that the predictions
based on the XRB modeling are about a factor 3 above the observed value at F10−40keV ∼
2× 10−11 erg cm−2 s−1, probably requiring a revision on the current XRB model’s
parameters (see also [18]).
At the SIMBOL-X flux limit of F10−40keV ∼ 7× 10−15 erg cm−2 s−1 (1 Msec ex-
posure) we predict a Compton Thick AGN surface density of 125 deg−2, i.e. around 5
Compton Thick AGN for each SIMBOL-X deep field. Thus with about 10 deep fields
we will be able to constrain the density of Compton Thick AGN with an error of about
15-20% in the redshift range between 0.8 and 1.5, where the peak of the redshift distri-
bution is expected (see Figure 1, right panel: z distribution). For a typical SIMBOL-X
exposure of 30 Ksec the flux limit (10-40 keV band) is expected to be ∼ 6× 10−14
erg cm−2 s−1. At this flux limit heavily obscured AGN can be assembled by looking
for serendipitous sources in pointed observations; considering a total number of ∼ 1000
SIMBOL-X pointings we should collect “for free" other ∼ 150 serendipitous Compton
Thick AGN spanning a redshift range from 0.05 to 0.6.
Finally it is worth noting that in the bright flux regime (few times above ∼ 10−13
erg cm−2 s−1), a complementary mission to SIMBOL-X is represented by the Energetic
X-ray Imaging Survey Telescope (EXIST; [10]) that is expected to perform an all sky
survey. At the "1 year survey flux limit" (5σ ) of F10−40keV ∼ 4×10−13 erg cm−2 s−1 the
predicted surface densities of Compton Thick AGN is∼ 0.1 deg−2 (see Figure 1, middle
panel), so ∼ 3000 Compton Thick AGN should be detected at high galactic latitude (the
large majority of them expected at z < 0.2). In this respect SIMBOL-X and EXIST are
synergic and not competing for what concerns the cosmological studies of the Compton
Thick AGN population: with EXIST it will be possible to derive the census (e.g. XLF)
of these objects in the local Universe while the much deeper surveys that will be carried
out with SIMBOL-X will be a fundamental and unique tool to study the evolution of
their XLF up to cosmological distances (z ∼ 2). As already stressed by [18] the study
of these surveys at E> 10 keV is the only way to constrain, on firm and solid statistical
bases, the census of heavily obscured accretion in AGN.
ACKNOWLEDGMENTS
We acknowledge financial support from the MIUR (grant PRIN-MIUR 2006-02-5203)
and from the Italian Space Agency (grant n. I/088/06/0 and COFIS contract).
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