Compton-thick AGN in the CDFN I. Georgantopoulos NOA A. Akylas NOA A. Georgakakis NOA M. Rovilos MPE...
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Transcript of Compton-thick AGN in the CDFN I. Georgantopoulos NOA A. Akylas NOA A. Georgakakis NOA M. Rovilos MPE...
Compton-thick AGN in the CDFN
I. Georgantopoulos NOAA. Akylas NOA
A. Georgakakis NOA
M. Rovilos MPE
M. Rowan-Robinson Imperial College
Why are Compton thick AGN important ?
X-ray surveys are extremely efficient in detecting AGN: the sky density of AGN is 5000 deg-2 (Bauer+04) orders of magnitude higher than that in the optical (eg Wolf+ 03)
However, at very high obscurations (NH~1024 cm-2 or AV>>100) even the hard 2-10 keV X-rays cannot penetrate these columns (Compton-thick AGN).
C-T are extremely important for the peak of the X-ray background at 40 keV (Gilli et al. 2007)
Spitzer mid-IR surveys are claiming the detection of AGN which are not detected in X-ray
The French/Italian Simbol-X, Japanese NeXT have as their main science goals the detection of these sources at very high energies.
Simbol-X
Present sample
Plot adapted from Gilli et al.
Talk Outline
A) X-ray spectral analysis in the CDF-N
B) X-ray Stacking analysis of mid-IR selected AGN not detected individually in the X-ray
X-ray spectral analysis
The sample
Selected all CDFN sources in the 2-10 keV band with flux > 1.e-15 cgs ie an order of mag above the flux limit
Two reasons: i) very good photon statistics spectra ii) Large number of redshifts (spec and photo)
222 sources of which 190 have redshift (107 spect) The photo-z have been derived using the code of Babbedge et al. using the IRAC Mags where available.
Median number of photons ~200
Method
C-T AGN can be detected either:
a) Absorption turn-over below 10 keV which shifts at lower energies at high redshifts because of K-correction
b) a flat spectrum Γ~1 (eg Matt 1996)
Spectral fits
a) single power-law model (WA*PO) with the NH free and Γ=1.8 b) single power-law model with the photon index free and the NH fixed
to the Galactic.
Then one can compare the values of chi-2 for the two models
We require Γ<1.2 (at the 90% upper limit) and Δχ2 > 5 corresponding to a probability value of
about 2σ that the ABSORBED model is better than the FLAT one [Likelihood ratio test Mushotzky 1982]
9 Compton thick sources
8 flat spectrum C-T candidates PLUS 1 for which we can see the spectral turnover directly at z=2
1 has no redshift (not in GOODS), 6/8 spectroscopic Redshifts
Mean redshift z=2.0
logLx=42-43 (observed)
Four of them are sub-mm sources (Alexander+2006)
ACS z-band cutouts of the C-T AGN
FeKα
The C-T logN-logS
Gilli predictions
Our points
Mid-IR selection
The Stern diagram: non X-ray detections
Is there a population
of mid-IR AGN
not detected in X-ray ?
Georgantopoulos+08 astro-ph/
AGN
GOODS IRAC sources
X-ray stacking analysis One can increase the effective exposure time by adding (stacking) the photons in areas of non-detected sources.
126 mid-IR ‘AGN’ at mean redshift z=1.4
Detection in both the soft (0.3-1.5 keV) and hard (1.5-4 keV) band but with a soft spectrum Γ~2.1 Moreover,
- Lx/LIR ~ 10-5
- Lx ~ 6x1040 cgs (soft band)
further supporting the galaxy scenario.
Optically-faint & red sources (a la Fiore selection)
Criteria:
R-[3.6] > 3.7 RED (EROs)
24 μm/R > 1000 optically FAINT
103 sources of which 20 detected in X-rays
The stacking analysis of the 83 gives Γ~0.8 or NH= 8x1022 cm-2 (@z=2) Lx/LIR ~ 10-5
These are either C-T or low luminosity absorbed AGN
Optically faint/ red sources: mid-IR diagram
Many sources outside the Stern wedge where the galaxy and Sy2 templates converge at z>2
X = x-ray detection
• = non-detection
ACS z-band cutouts of the X-ray detections in the selection ‘a la FioreGreen circle = 3.6 micron position
X-ray Absorbed sources are associated with early-type systems at z~1. Does the same hold at higher
redshifts ? X-ray obscured sources are RED at z~1Rovilos & Georgantopoulos 07
(ECDFS)
Georgakakis+08 (AEGIS)
See also Silverman+07
is there another tendency at higher z ?
RemarksCompton-thick candidate AGN can be easily detected through X-ray spectroscopy. XMM observations could then play the major role
Advertisement of the 1.3 CDFS XMM observation (PI Comastri) of the CDFS which will provide many bona-fide C-T AGN at faint fluxes
At brighter fluxes the fraction of C-T AGN is >>1/100. Analysis of the whole 2XMM catalogue should produce at least a few tens
In the future all the pieces of the puzzle will be filled at faint 2-10 keV fluxes with XEUS and observations at hard energies with Simbol-X and NEXT
THE END
Stern diagram : X-ray detections Stern+05 wedge is a good
way to sift mid-IR AGN
But not all X-ray AGN fall into the Stern wedge
see also Barmby+06